Reference Levels of Insulin-Like Growth Factor I in the Serum of Healthy Adults: Comparison of Four Immunoassays

IGF-1 as screening tool for acromegaly and adult-onset growth hormone deficiency in the Netherlands

OBJECTIVE

Insulin-like growth factor 1 (IGF-1) measurements play a central role in the diagnosis and follow-up of acromegaly and growth hormone deficiency. However, improving health care outcomes for these patients involves an intricate process of laboratory diagnostics and skilled health care professionals. The integrated effects of IGF-1 reports on diagnosis and treatment decisions are yet unknown.

DESIGN, PATIENTS AND MEASUREMENTS

Extended quality assessment, distributing the description of five (real) patient cases with accompanying blood samples. Patients suspected or during follow up for acromegaly or adult onset of growth hormone deficiency were included. Laboratory specialists and endocrinologists in the same centre were asked to interpret their centre-specific IGF-1 results by using a laboratory and medical questionnaire. This way, insight could be obtained into the combined effects of different assays, assay harmonisation, reference value sets, and individual physician interpretation in relation to guidelines, thus reviewing the entire diagnostic and management process.

RESULTS

Limited variation (CV 13.8 ± 2.8) was found in IGF-1 concentrations despite different use of the harmonization sample and factor among laboratories. This interlaboratory variation increased upon conversion to SD scores (CV 15.7 ± 40.7) as a consequence of the use of different reference value sets. Furthermore, there was a lack of adherence to international guidelines among endocrinologists.

CONCLUSIONS

Highly variable diagnostic and treatment outcomes in acromegaly and AGHD in the Netherlands can be attributed to increased variability of IGF-1 upon conversion to SD scores and low adherence to clinical guidelines.

The impact of different calibration matrices on the determination of insulin-like growth factor 1 by high-resolution-LC-MS in acromegalic and growth hormone deficient patients

OBJECTIVES

Calibration is an important source of variability in liquid chromatography mass spectrometry (LC-MS) methods for insulin-like growth factor 1 (IGF-1). This study investigated the impact of different calibrator matrices on IGF-1 measurements by LC-MS. Moreover, the comparability of immunoassays and LC-MS was assessed.

DESIGN & METHODS

Calibrators from 12.5 to 2009 ng/ml were prepared by spiking WHO international Standard (ID 02/254 NIBSC, UK) into the following matrices: native human plasma, fresh charcoal-treated human plasma (FCTHP), old charcoal-treated human plasma, deionized water, bovine serum albumin (BSA), and rat plasma (RP). A validated in-house LC-MS method was calibrated repeatedly with these calibrators. Then, serum samples from 197 growth hormone excess and deficiency patients were analysed with each calibration.

RESULTS

The seven calibration curves had different slopes leading to markedly different patient results. The largest differences in IGF-1 concentration from the median (interquartile range) was observed with the calibrator in water and the calibrator in RP (336.4 [279.6-417.0] vs. 112.5 [71.2-171.2], p < 0.001). The smallest difference was observed with calibrators in FCTHP and BSA (141.8 [102.0-198.5] vs. 127.9 [86.9-186.0], p < 0.049). Compared to LC-MS with calibrators in FCTHP, immunoassays showed relevant proportional bias (range: -43% to -68%), constant bias (range: 22.84 to 57.29 ng/ml) and pronounced scatter. Comparing the immunoassays with each other revealed proportional bias of up to 24%.

CONCLUSIONS

The calibrator matrix is critical for the measurement of IGF-1 by LC-MS. Regardless of the calibrator matrix, LC-MS shows poor agreement with immunoassays. Also, the agreement between different immunoassays is variable.

Growth Hormone as Biomarker in Heart Failure

The impairment of growth hormone (GH)/insulin growth factor-1(IGF-1) plays a crucial role in chronic heart failure (CHF). Several studies have shown that patients affected by this condition display a more aggressive disease, with impaired functional capacity and poor outcomes. Interestingly, GH replacement therapy represents a possible future therapeutic option in CHF. In this review, the authors focus on the assessment of the main abnormalities in GH/IGF-1 axis in CHF, the underlying molecular background, and their impact on disease progression and outcomes.

Utility of serum IGF-1 for diagnosis of growth hormone deficiency following traumatic brain injury and sport-related concussion

BACKGROUND

Growth hormone deficiency (GHD) is a potential consequence of traumatic brain injury (TBI), including sport-related concussion (SRC). GH stimulation testing is required for definitive diagnosis; however, this is resource intensive and can be associated with adverse symptoms or risks. Measurement of serum IGF-1 is more practical and accessible, and pituitary tumour patients with hypopituitarism and low serum IGF-1 have been shown to have a high probability of GHD. We aimed to evaluate IGF-1 measurement for diagnosing GHD in our local TBI population.

METHODS

We conducted a retrospective chart review of patients evaluated for GHD at the TBI clinic and referred for GH stimulation testing with insulin tolerance test (ITT) or glucagon stimulation test (GST) since December 2013. We obtained demographics, TBI severity, IGF-1, data pertaining to pituitary function, and GH stimulation results. IGF-1 values were used to calculate z-scores per age and gender specific reference ranges. Receiver operator curve analysis was performed to evaluate diagnostic threshold of IGF-1 z-score for determining GHD by GST or ITT.

RESULTS

Sixty four patient charts were reviewed. 48 patients had mild, six had moderate, eight had severe TBI, and two had non-traumatic brain injuries. 47 patients underwent ITT or GST. 27 were confirmed to have GHD (peak hGH < 5 μg/L). IGF-1 level was within the age and gender specific reference range for all patients with confirmed GHD following GH stimulation testing. Only one patient had a baseline IGF-1 level below the age and gender specific reference range; this patient had a normal response to GH stimulation testing. ROC analysis showed IGF-1 z-score AUC f, confirming lack of diagnostic utility.

CONCLUSION

Baseline IGF-1 is not a useful predictor of GHD in our local TBI population, and therefore has no value as a screening tool. TBI patients undergoing pituitary evaluation will require a dynamic test of GH reserve.

Agreement between circulating IGF-I, IGFBP-1 and IGFBP-3 levels measured by current assays versus unavailable assays previously used in epidemiological studies

OBJECTIVE

Levels of insulin-like growth factor (IGF) proteins are associated with the risk of cancer and mortality. IGF assays produced by Diagnostic Systems Laboratories (DSL) were widely used in epidemiological studies, were not calibrated against recommended standards and are no longer commercially available.

DESIGN

In a split sample study among 1471 adults participating in the Cardiovascular Health Study, we compared values obtained using DSL assays with alternative assays for serum IGF-I (Immunodiagnostic Systems, IDS), IGFBP-1 (American Laboratory Products Company, ALPCO) and IGFBP-3 (IDS).

RESULTS

Results were compared using kernel density estimation plots, quartile analysis with weighted kappa statistics and linear regression models to assess the concordance of data from the different assays. Participants had a mean age of 77years. Results between alternative assays were strongly correlated (IGF-I, r=0.93 for DSL versus IDS; log-IGFBP-1, r=0.90 for DSL versus ALPCO; IGFBP-3, r=0.92 for DSL versus IDS). Cross tabulations showed that participants were usually in the same quartile categories regardless of the assay used (overall agreement, 74% for IGF-I, 64% for IGFBP-1, 71% for IGFBP-3). Weighted kappa also showed substantial agreement between assays (kw, 0.78 for IGF-I, 0.69 for IGFBP-1, 0.76 for IGFBP-3). Regressions of levels obtained with DSL assays (denoted X) to alternative assays were, IGF-I: 0.52X+15.2ng/ml, log-IGFBP-1: 1.01X-1.73ng/ml IGFBP-3: 0.87X+791.1ng/ml. Serum values of IGF-I, IGFBP-1 and IGFBP-3 measured using alternative assays are moderately correlated.

CONCLUSIONS

Care is needed in the interpretation of data sets involving IGF analytes if assay methodologies are not uniform.

Effect of a 4-week weight maintenance diet on circulating hormone levels: implications for clinical weight loss trials

The majority of weight loss studies fail to standardize conditions such as diet and exercise via a weight maintenance period prior to commencement of the trial. This study aimed to determine whether a weight stabilization period is necessary to establish stable baseline hormone concentrations. Fifty-one obese male participants with a body mass index of 30-40 kg m(-2) and aged 25-54 years underwent 4 weeks on an energy balance diet that was designed to achieve weight stability. Blood samples were collected in the fasting state at commencement and completion of the 4-week period, and circulating concentrations of 18 commonly measured hormones were determined. During the 4-week weight maintenance period, participants achieved weight stability within -1.5 ± 0.2 kg (-1.4 ± 0.2%) of their initial body weight. Significant reductions in serum insulin (by 18 ± 6.5%) and leptin (by 21 ± 6.0%) levels occurred, but no significant changes were observed for gut-derived appetite-regulating hormones (ghrelin and peptide YY), nor thyroid, adrenal, gonadal or somatotropic hormones. There were no significant correlations between the change in body weight and the change in circulating concentrations of insulin or leptin over the 4-week period, indicating that the observed changes were not due to weight loss, albeit significant negative correlations were observed between the changes in body weight and plasma ghrelin and peptide YY levels. This study demonstrates the need for baseline weight maintenance periods to stabilize serum levels of insulin and leptin in studies specifically investigating effects on these parameters in the obese. However, this does not apply to circulating levels of gut-derived appetite-regulating hormones (ghrelin and peptide YY), nor thyroid, adrenal, gonadal or somatotropic hormones.

Reference values of IGF1, IGFBP3 and IGF1/IGFBP3 ratio in adult population in the Czech Republic

BACKGROUND

IGF1 is responsible for regulation of growth, metabolism and differentiation of human cells. IGFBP3 is the most abundant of the carrier proteins for IGF1 in the blood. IGF1/IGFBP3 molar ratio is an indicator of IGF1 bioavailability. We decided to create a file of reference ranges of IGF1, IGFBP3 and IGF1/IGFBPP3 ratio for the adult Czech population across the age spectrum.

METHODS

We selected a group of 1022 subjects, 467 males and 555 females (ages 20-98 years), from several regions in the Czech Republic. The group consisted of blood donors and patients undergoing regular preventive examinations. Serum levels of IGF1 and IGFBP3 were measured using the following radioimmunoassay kits: IRMA IGF1 (Immunotech, Marseille, France) and IRMA IGFBP3 (Immunotech, Prague, Czech Republic). The IGF1/IGFBP3 ratio was also calculated. The following groups of patients were excluded: patients with diabetes, high blood glucose, high insulin levels, post-surgery patients, polymorbid patients, and subjects with oncological diseases. Subjects were divided into seven age-groups. Changes in the levels of observed analytes in each decade across the age spectrum were evaluated. All statistical analyses were performed by SAS 9.3 (Statistical Analysis Software release 9.3; SAS Institute Inc., Cary, NC, USA).

RESULTS

All three parameters IGF1, IGFBP3 and IGF1/IGFBP3 decreased in parallel with decrease in age: p<0.0001, r=-0.64, -0.35 and -0.54, respectively. The dynamics of the decline was different between males and females. Linear regression models with age as independent variable fitted by gender are displayed in Fig. 1. Non-parametric reference interval curves (medians and 2.5th-97.5th percentiles) for IGF1, IGFBP3 and IGF1/IGFBP3 ratio as function of age by gender are displayed in Fig. 2(a,b,c). All medians and 2.5th-97.5th percentiles were plotted by cubic spline. For males, linear regression models were as follows: IGF1=291.34619-2.41211 × age, IGFBP3=2931.62778-6.11659 × age, IGF1/IGFBP3=0.02897-0.00021213 × age. For females, we plotted the following: IGF1=241.67406-1.98466 × age, IGFBP3=3688.60561-16.39560 × age, IGF1/IGFBP3=0.02029-0.00013233 × age. IGF1 was statistically significantly higher in males with p<0.0001 (Wilcoxon test) but decreased faster (p=0.0121). IGFBP3 was statistically significantly higher in females with p=0.0004 (Wilcoxon test) but decreased faster (p<0.0001). IGF1/IGFBP3 was statistically significantly higher in males with p<0.0001 (Wilcoxon test) but decreased faster (p<0.0001).

CONCLUSION

Authors recommend using of a linear regression model based reference ranges for IGF1, IGFBP3 and IGF1/IGFBP3 ratio and using different reference ranges for genders.

The trajectory of IGF-1 across age and duration of type 1 diabetes

BACKGROUND

Individuals with type 1 diabetes may have low IGF-1, related to insulinopenia and insulin resistance. There are few longitudinal studies of IGF-1 levels to establish its pattern in type 1 diabetes with duration and age, and to examine whether IGF-1 tracks within individuals over time. We examine age and duration trends, and the relationship of IGF-1 to gender, glycaemic control, insulin level and other factors.

METHODS

Participants in the Wisconsin Diabetes Registry Study, an incident cohort study of type 1 diabetes diagnosed May 1987-April 1992, were followed for up to 18 years with IGF-1 samples up to age 45 for women and age 37 for men.

RESULTS

IGF-1 is lower with type 1 diabetes than in normative samples. Although, the pattern across age resembles that in normative samples with a peak in adolescence and slow decline after age 20, the adolescent peak is delayed for women with type 1 diabetes. There was low to moderate tracking of IGF-1 within an individual. Higher insulin dose was associated with higher IGF-1 as was puberty, and female gender. Adjusted for these factors, IGF-1 declined rapidly across early diabetes duration. Lower HbA1c was most strongly related to higher IGF-1 at Tanner stages 1 and 2.

CONCLUSIONS

IGF-1 is low in type 1 diabetes, with a delayed adolescent peak in women and is especially influenced by glycaemic control in early and pre-adolescence. High variability within an individual is likely a challenge in investigating associations between IGF-1 and long-term outcomes, and may explain contradictory findings.

Analytical challenges in the detection of peptide hormones for anti-doping purposes

Although significant progress has been achieved during the past few years with the introduction of new assays and analytical methodologies, the detection and quantification of protein analytes, in particular of peptide hormones, continues to pose analytical challenges for the World Anti-Doping Agency-accredited anti-doping laboratories. In this article, the latest achievements in the application of MS-based methodologies and specific biochemical and immunological assays to detect some of the prohibited substances listed in section S2 of the World Anti-Doping Agency List of Prohibited Substances and Methods are reviewed. In addition, we look towards the future by focusing on some of the most promising analytical approaches under development for the detection of so-called ‘biomarkers of doping’.

Epidemiologic survey: reference ranges of serum insulin-like growth factor 1 levels in Caucasian adult population with immunoradiometric assay

Gender, age adjusted, population based reference ranges are necessary to use insulin-like growth factor 1 (IGF-1) as a diagnostic marker or for therapeutic monitoring in growth hormone (GH) related diseases. The aim of the present study was to describe the serum IGF-1 distribution and to calculate age and gender specific reference values for Caucasian adult population. A representative sample of 1002 male and 1039 female, totally 2041 participants aged above 18 years old was examined. The subjects suffering from diabetes mellitus, renal diseases, liver diseases, cancer, or diseases of pituitary gland were excluded by medical history, physical examination, and laboratory tests. The subjects were not using any drug that could affect IGF-1 levels. Body mass index (BMI)>30 or<18 kg/m2 were excluded. Serum IGF-1 concentrations were determined by immunoradiometric assay (IRMA). Serum IGF-1 concentrations were declined with age in both males and females after the age of 18. Males had significantly higher serum IGF-1 levels than females in the age groups 18-24, 50-69 (P<0.05), but not in others (P>0.05). The present study established age and gender specific reference ranges for serum IGF-1 levels calculated for Caucasian adult population with IRMA that could be used in medical practice.

Plasma insulin-like growth factor I as predictor of progression and all cause mortality in chronic heart failure

OBJECTIVES

Insulin-like growth factor I (IGF-I) is an anabolic growth factor that seems to increase cardiac contractility. Reduced levels of IGF-I may be implicated in progression of CHF. The objective was to compare plasma IGF-I in CHF patients with healthy controls, and to examine the associations between baseline IGF-I levels, cardiac contractility and the prognosis as judged by all cause mortality and progression of CHF requiring admission to hospital.

METHODS

A prospective study comprising 194 CHF outpatients, and 169 matched controls. All patients and controls underwent echocardiographic examination at baseline. Patients were followed for a median of 30 months.

RESULTS

There was no difference in IGF-I levels between patients and controls (median and interquartile range), 78 (58-91) vs. 77 (57-94)ng/mL (P=0.92). Age-adjusted IGF-I levels were not related to left ventricular ejection fraction (LVEF) (P=0.58) or levels of N-terminal B-Type natriuretic peptide (NT-proBNP) (P=0.42). During follow-up 44 patients died and 94 were admitted to hospital due to worsening of CHF. Adjusted for cardiovascular risk factors (age, gender, NT-proBNP, lipids, diabetes mellitus, blood pressure, renal function and LVEF) IGF-I levels did not influence the overall mortality risk or the admission rate to hospital, hazard ratio (HR) (95% confidence intervals) 1.05 (0.75-1.47) (P=0.77) and 1.00 (0.80-1.26) (P=0.96), respectively per each SD increase in log IGF-I levels.

CONCLUSIONS

IGF-I levels were not reduced in patients with CHF and did not influence cardiac status at baseline or the prognosis.

Five commercially available insulin-like growth factor I (IGF-I) assays in comparison to the former Nichols Advantage IGF-I in a growth hormone treated population

BACKGROUND

The serum insulin-like growth factor I (IGF-I) level is accepted to diagnose the growth hormone (GH) status. Here, we evaluated the DRG IGF-I 600 ELISA, DSL IGF-I ELISA, IDS OCTEIA IGF-I, Mediagnost IGF-I-ELISA, and the Siemens Immulite 2500 IGF-I in comparison to the former Nichols Advantage IGF-I assay.

METHODS

Imprecision was determined by use of a serum pool and commercial control materials. Accuracy was evaluated by means of a method comparison to Nichols in 173 serum samples of GH deficient patients.

RESULTS

The Siemens and the IDS IGF-I assays showed the lowest imprecision with coefficients of variation up to 3.6% and 6.9%, respectively. Both correlated best to Nichols (Siemens: y=0.667X+8.8 microg/L, r=0.950; IDS: y=0.527 X+4.6 microg/L, r=0.927) with the lowest dispersion of residuals from a linear equation. The DSL assay had the highest comparability to Nichols (y=1.000 X+35.5 microg/L, r=0.864), but with a considerable scattering.

CONCLUSIONS

To yield IGF-I determination comparable to the former Nichols IGF-I, either the Siemens or the IDS assay should be applied, and the results should be converted by a linear method transformation. Where a conversion factor is not desired, the DSL assay should be selected.

Multidimensional reference regions for IGF-I, IGFBP-2 and IGFBP-3 concentrations in serum of healthy adults

CONTEXT

The insulin-like growth factor (IGF) system exerts many effects on the growth and differentiation of both normal and malignant cells. The serum concentrations of insulin-like growth factor I (S-IGF-I), insulin-like growth factor-binding protein 2 (S-IGFBP-2) and insulin-like growth factor-binding protein 3 (S-IGFBP-3) and their inter-relations may differ in certain disease states from those seen in healthy individuals.

OBJECTIVE

To estimate age-, gender- and body mass index (BMI)-specific univariate, bivariate and trivariate 95% reference regions for these components in healthy adults and present indices that will facilitate interpretation of patient observations in relation to these reference regions.

DESIGN

Blood samples were taken in the morning from 427 healthy, non-fasting German blood donors of both genders (age range, 18-79 years; BMI range, 16-44 kg/m(2)). Reference regions were developed with multivariate regression methods.

RESULTS

Regression analyses showed that S-IGF-I and S-IGFBP-3 levels decrease with increasing age, whereas S-IGFBP-2 concentrations increase with age (P<0.0001). Females had significantly higher S-IGFBP-3 levels than males (P<0.0001) and similar S-IGF-I and S-IGFBP-2 concentrations. Increasing BMI was associated with decreasing S-IGFBP-2 (P<0.0001), but was not significantly associated with the concentrations of the other two analytes. Controlling for age, gender and BMI, S-IGF-I and S-IGFBP-3 were positively correlated (r=0.58), whereas S-IGF-I and S-IGFBP-2, and S-IGFBP-2 and S-IGFBP-3 were negatively correlated (r=-0.11 and r=-0.18, respectively). Based on the regression models, which were controlled for age, gender and BMI, two- and three-dimensional 95% reference regions with associated patient indices were described for each pair of analytes and for the trio of analytes, respectively.

CONCLUSIONS

The multivariate reference regions developed in this study should be useful in identifying patients with an abnormal IGF-IGFBP axis. It is suggested that introducing multidimensional reference regions and the described patient indices into clinical practice may aid monitoring of the safety of GH therapy. These patient indices may also be useful in the assessment of cancer risk.

Normal values of circulating insulin-like growth factor-I bioactivity in the healthy population: comparison with five widely used IGF-I immunoassays

BACKGROUND

IGF-I immunoassays are primarily used to estimate IGF-I bioactivity. Recently an IGF-I-specific kinase receptor activation assay (KIRA) has been developed as an alternative method. However, no normative values have been established for the IGF-I KIRA.

OBJECTIVE

The objective of the study was to establish normative values for the IGF-I KIRA in healthy adults.

DESIGN

This was a cross-sectional study in healthy nonfasting blood donors.

STUDY PARTICIPANTS

Participants included 426 healthy individuals (310 males, 116 females; age range 18-79 yr).

MAIN OUTCOME MEASURES

IGF-I bioactivity determined by the KIRA was measured. Results were compared with total IGF-I, measured by five different IGF-I immunoassays.

RESULTS

Mean (+/- sd) IGF-I bioactivity was 423 (+/- 131) pmol/liter and decreased with age (beta = -3.4 pmol/liter.yr, P < 0.001). In subjects younger than 55 yr, mean IGF-I bioactivity was significantly higher in women than men. Above this age this relationship was inverse, suggesting a drop in IGF-I bioactivity after menopause. This drop was not reflected in total IGF-I levels. IGF-I bioactivity was significantly related to total IGF-I (r(s) varied between 0.46 and 0.52; P < 0.001).

CONCLUSIONS

We established age-specific normative values for the IGF-I KIRA. We observed a significant drop in IGF-I bioactivity in women between 50 and 60 yr, which was not perceived by IGF-I immunoassays. The IGF-I KIRA, when compared with IGF-I immunoassays, theoretically has the advantage that it measures net effects of IGF-binding proteins on IGF-I receptor activation. However, it has to be proven whether information obtained by the IGF-I KIRA is clinically more relevant than measurements obtained by IGF-I immunoassays.

Reference ranges of serum IGF-1 and IGFBP-3 levels in a general adult population: results of the Study of Health in Pomerania (SHIP)

OBJECTIVE

Insulin-like growth factor I (IGF-1), which is mostly carried by IGF binding protein 3 (IGFBP-3), mediates endocrine actions of growth hormone and represents an important prognostic factor for systemic diseases. The main objectives of the present study were (1) to calculate age- and sex-specific reference values for serum IGF-1 and IGFBP-3 levels using the Gaussian mean+/-1.96 *standard deviation method and the quantile regression method and (2) to perform a comparison of the results from both statistical methods.

DESIGN

Cross-sectional study (Study of Health in Pomerania). The study population included 2499 men and women aged 20-79 years.

METHODS

Serum IGF-1 and IGFBP-3 levels were determined by chemiluminescence immunoassays. Linear and quantile regressions were performed to calculate age- and sex-related reference ranges.

RESULTS

Both statistical methods generated different results: The reference ranges based on linear regression identified 43 (1.7%) subjects with serum IGF-1 levels and 99 (4.0%) subjects with serum IGFBP-3 levels outside the reference range. Using quantile regression, 117 (4.7%) or 118 subjects (4.7%) with serum IGF-1 or IGFBP-3 levels outside the range were detected, respectively.

CONCLUSION

The present study established age- and sex-specific reference ranges for serum IGF-1 and IGFBP-3 levels. Quantile regression should be preferred to calculate reference ranges because a better concordance to original data is possible due to no distribution assumption are required and the robustness against outliers is given.

Serum IGF-I measured by four different immunoassays in patients with adult GH deficiency or acromegaly and in a control population

BACKGROUND

IGF-I is a useful tool in GH disorders diagnosis, however, the use of commercially available kits needs to be validated.

OBJECTIVE

To validate the use of serum IGF-I concentrations measured by four immunoassays in the diagnosis of adult GH deficiency and acromegaly.

DESIGN

Cross-sectional study.

PATIENTS

Fifty GH-deficient (GHD) patients, 41 acromegaly patients and 405 controls.

MEASUREMENTS

Serum IGF-I concentrations were measured by four commercial immunoassays: (1) RIA-NICHOLS; (2) ICMA-IMMULITE; (3) IRMA-IMMUNOTECH; and (4) non-extraction-IRMA-DSL. Reference values were established from the control population in six age groups. Individual results were transformed to standard deviation score (SD score) from the age-related reference population and reference data provided by each assay manufacturer. Diagnostic sensitivity for GH deficiency was calculated.

RESULTS

IGF-I measured by the four assays differed significantly. In controls, assay 2 yielded the lowest results, followed by assays 1, 3 and 4 (P < 0.0001 for all comparisons). IGF-I declined with age, but no sex-related differences were observed. When IGF-I was standardized with respect to reference data obtained from the manufacturers, it showed better sensitivity in assays 1 and 2, than with our controls (65%vs. 77.5% and 58%vs. 70%, respectively) for GHD diagnosis. With assays 3 and 4, higher sensitivity was obtained when standardized with our controls (62%vs. 52% and 56%vs. 36%, respectively). In acromegaly, IGF-I was > 2 SD score with all assays.

CONCLUSIONS

IGF-I SD score for GHD diagnosis differed according to the normative data used. All assays proved to be useful for active acromegaly diagnosis.

Normal age-dependent values of serum insulin growth factor-I: results from a healthy Italian population

Serum IGF-I levels were measured in 547 non-hypopituitaric, non-acromegalic healthy subjects of both sexes in Italy to develop reference values in relation to age and sex. Participant subjects were stratified in three age classes (25- 39, 40-59 and >or=60 yr) and IGF-I assay was carried out by double-antibody radio immunoassay. Pearson's correlation coefficient between age and IGF-I values was calculated by sex and predefined age ranges. IGF-I levels significantly decreased with age (p<0.001, Kruskal-Wallis test) while sex was not a significant factor. The median IGF-I levels were 206 ng/ml in the 25-39 yr range, 147 ng/ml in the 40-59 yr range and 103 ng/ml in the >or=60 yr range. Pearson's correlation coefficient confirmed the negative correlation between age and IGF-I levels in the total sample of subjects (r=-0.529). The r coefficient between age and IGF-I levels did not differ between sexes (r=-0.570 in males and r=-0.529 in females), thus reflecting no sex-effect on IGF-I levels decline over years. No correlations were found in the 25-39 yr range (r=-0.036) or in the 40-59 yr range (r=-0.080) either, while in subjects aged >60 yr, IGF-I levels tended to further decrease with increased age (r=0.389). Ranges of normal values set at the 2.5th-97.5th percentile in the three age ranges were 95.6-366.7 ng/ml between 25 and 39 yr, 60.8-297.7 ng/ml between 40 and 59 yr and 34.5-219.8 ng/ml in subjects aged >or=60 yr. This study may contribute to the development of age-specific reference ranges for IGF-I determination in serum of normal subjects of both sexes in Italy.

Central laboratory reassessment of IGF-I, IGF-binding protein-3, and GH serum concentrations measured at local treatment centers in growth-impaired children: implications for the agreement between outpatient screening and the results of somatotropic axis functional testing

BACKGROUND

Childhood GH deficiency, suspected in the presence of decreased height velocity and short stature, is usually characterized by low IGF-I and IGF-binding protein-3 (IGFBP-3) serum concentrations and is conventionally confirmed by diminished GH peak responses to pharmacological stimuli.

OBJECTIVE

We evaluated the agreement between different IGF-I (IGFBP-3) assays in predicting GH deficiency and tested whether variability between growth factor screening and pharmacological testing could be diminished by reassessment of growth factor and GH peak concentrations in a single laboratory.

DESIGN

Using the Tuebingen IGF-I (IGFBP-3) RIA, 317 (321) sera from children evaluated for growth disorders in 19 centers were reanalyzed. In 103 children with insulin hypoglycemia and arginine tests, we evaluated how the association between the outcome of growth factor screening and functional testing would change if different assays were employed.

RESULTS

Locally measured IGF-I correlated better than IGFBP-3 with the results of the central laboratory (Tuebingen) assay (slope of the regression curve 1.05; 95% confidence interval (95% CI) 1.01-1.1 versus 1.18; 95% CI 1.09-1.3). Agreement between local and central laboratory assays in predicting GH deficiency was better for IGF-I than for IGFBP-3 assays (kappa =0.59 versus kappa =0.47). The poor agreement between growth factor screening and GH pharmacological testing was not improved when hormone concentrations were remeasured in the central laboratory (kappa local=-0.0031, central=0.12).

CONCLUSIONS

In children with impaired growth, growth factor screening reflects different aspects of GH insufficiency than does functional testing. Agreement between these approaches is poor and could not be improved by reduction of assay-related variability.

Serum insulin-like growth factor I and ischemic brain injury

Serum insulin-like growth factor I (IGF-I), which is mostly produced by the liver, has recently been shown to have the unexpected ability to modulate normal brain function as well as brain response to injury. Moreover, serum IGF-I levels are modified in many brain diseases, including stroke. However, whether these modifications are related to the disease process remains uncertain. We now examined a potential relationship between serum IGF-I and ischemic brain injury after middle cerebral artery occlusion (MCAo) and reperfusion in mice with either high or low serum IGF-I levels prior to insult. Surprisingly, we found that chronic high serum IGF-I correlates with increased brain infarct size following MCAo, while low levels correlate with reduced lesion size. Immunocytochemistry and immunoblot analyses revealed that levels of phosphorylated (i.e., activated) MAPK, known to be associated with the severity of ischemic brain injury, were increased in IGF-I treated mice. No overall effect of IGF-I treatment on IGF family mRNA expression in the brain was observed. Altogether, these results indicate that serum IGF-I levels negatively correlate with stroke outcome. Therefore, lowering serum IGF-I levels in aging mammals, including humans, may be beneficial against the increased risk of stroke associated to old age.

IGF-I assays: current assay methodologies and their limitations

The diagnosis of disorders of growth hormone (GH) is dependent upon accurate measurement of insulin-like growth factor-I (IGF-I) concentrations since serum IGF-I assays have been found to be useful as a screening tests for the presence of growth hormone deficiency (GHD) in children and in both children and adults they have been found very useful in establishing the diagnosis of acromegaly. IGF-I is also used extensively to monitor the response to GH treatment in children and adults and to monitor the response to treatment in acromegaly. Since IGF-I is influenced by several other hormones and physiologic factors as well as GH, a knowledge of its regulation is essential to understanding how to properly interpret the measurements. Several technical criteria are required for successful laboratory estimation of IGF-I values. These include elimination of interference of IGF-I-binding proteins (IGFBP), utilization of adequate numbers of normal subjects to define the normal ranges and importantly the use of high affinity, high specificity antisera that allow precise and reproducible measurements of the biologically active peptide. Cross comparisons of various commercial assays show that the results generally are similar when values are in the normal range. However, the assays have different performance characteristics when concentrations are either above or below the normal range. To obtain cross laboratory standardization for values outside the normal range requires utilization of similar, high-quality reagents and techniques that are reasonably comparable. Without this degree of standardization, cross comparisons among various reference laboratories are likely to continue to show wide divergence for values that are above or below the 95% confidence interval. A future goal should be the development of standard procedures and reagents that eliminate this degree of variability.

Normal levels of serum IGF-I: determinants and validity of current reference ranges

Insulin like growth factor I (IGF-I) represents the key marker for the evaluation of Growth hormone (GH) status. As a large number of determinants including age, gender, genetic factors, nutrition, and disease states influence IGF-I serum levels, accurate normative data are essential to translate patient data into diagnostic meaning or even use IGF-I levels for adequate monitoring of patients with an over- or under-active GH axis. Even though reference ranges have been developed in large cohorts of healthy subjects, the dependency of these data on a given assay technology argues for assay specific normative data for IGF-I.

Variability among five different commercial IGF-1 immunoassays in conditions of childhood-onset GH deficiency and GH therapy

IGF-1 measurement is used for screening of GH deficiency and monitoring of GH therapy in children. However, several commercial immunoassays are currently used and reference values provided by manufacturers are very different. The aim of this study was to compare commercial IGF-1 assays 1) in terms of absolute values and 2) in terms of clinical interpretation of results based on IGF-1 reference values in serum samples from children with GH therapy, with untreated GH deficiency and with obesity. Serum samples of 9 patients were sent frozen to 5 university hospitals using 5 different IGF-1 assays. The inter-laboratory coefficient of variation (CV) was calculated for the 9 samples. For clinical interpretation, results were expressed as SD scores based on reference values provided by manufacturers (and used in these laboratories). The mean inter-laboratory CV (range) for the 9 serum samples was 25.8% (16.7-35.9%). Major variability was noted in the SD-scores between IGF-1 assays for 3 tested serum samples from GH-treated patients with a difference between the lowest and highest SD score of 2.6 up to 3.2. In conclusion, there is a large variability among commercial IGF-1 immunoassays, not only in terms of absolute values, but also in terms of clinical interpretation in pediatric serum samples. There is a need for IGF-1 immunoassay harmonization and for the establishment of adequate reference values.

Insulin-Like Growth Factor I Levels in Healthy Adults

Insulin-like growth factor I (IGF-I) levels mainly reflect secretion of growth hormone (GH) in the body. The aims of this study were to compare different IGF-I assay methods in healthy individuals, test the reliability of the methods and discuss the utility of IGF-I measurement in adults. The Nichols Institute Diagnostics radioimmunoassay was used to evaluate IGF-I in two random population samples of men and women (aged 25-64 years, n = 392) taken 10 years apart, in 1985 and 1995. This method for IGF-I testing was also compared with an immunoradiometric assay (IRMA) method in 387 men and women participating in the World Health Organization MONICA (MONItoring of trends and determinants for CArdiovascular diseases) Project, Goteborg, Sweden, in 1995. Serum IGF-I decreased with increasing age in both men and women. IGF-I was higher in young women compared with young men in both cohorts, while the opposite was found in the highest age group. Age-adjusted significant correlations were found between IGF-I and smoking, fibrinogen, coffee consumption, lipoprotein (a), osteocalcin and IGF-binding protein 3. The two cohorts showed similar mean IGF-I concentrations irrespective of method. The correlation between the Nichols and the IRMA methods was high: r = 0.93 (p < 0.0001). Based on this and previous studies, population-based IGF-I measurements are robust irrespective of which commercially available method of assay is used. IGF-I levels can be used in diagnosing acromegaly as well as providing target values. IGF-I assay can be used as a complement to stimulation testing in the diagnosis of GH deficiency, and as a tool for GH dose titration.