Development of a Model to Estimate 24-Hour Urinary Creatinine Excretion

Feasibility of 24-h urine creatinine clearance as a renal function monitoring tool in spinal cord injury patients

OBJECTIVE

Renal dysfunction is a major cause of morbidity in patients with spinal cord injury (SCI). A 24-h urine creatinine (Cr) clearance (24-h urine CCr) is cost-effective and easy to implement compared to renal scintigraphy in the evaluation of renal function. This study aimed to verify the feasibility of 24-h urine CCr in the SCI population by assessing the correlation with effective renal plasma flow (ERPF) on renal scintigraphy.

METHODS

Data from 245 SCI patients (189 males, mean age: 50.2 years) were used in this retrospective review. Clinical characteristics, 24-h urine CCr, serum Cr, comorbidities, and body composition analyses were assessed for correlation with laboratory parameters including renal scintigraphy. Strong predictors of ERPF were determined by multivariate linear regression analysis. Areas under receiver-operating characteristic curves were calculated to evaluate the discriminating power of 24-h urine CCr to predict ERPF <250 ml/min.

RESULTS

Spinal cord injury patients showed tubular dysfunction despite normal serum Cr and 24-h urine CCr. There was a significant correlation between 24-h urine CCr and ERPF, and 24-h urine CCr was one of the strongest predictors for ERPF (area under the curve 0.72, 95% CI 0.64-0.80, p < 0.000) among other parameters such as age, appendicular lean mass index, and body mass index. 24-h urine CCr was an independent predictor of ERPF in subacute (R²  = 0.497, p < 0.001) and chronic SCI patients (R²  = 0.664, p < 0.0001). The optimized 24-h urine CCr cut-off was 139.4 ml/min/1.72 m² for predicting decreased ERPF <250 ml/min (sensitivity 67.6% and specificity 64.0%).

CONCLUSION

24-h urine CCr is a sensitive indicator for renal function deterioration of SCI patients. Further longitudinal studies with larger numbers of SCI patients are needed to confirm the feasibility of 24-h urine CCr for monitoring this population.

Estimating urine volume from the urine creatinine concentration

Spot determinations of the urine creatinine concentration are widely used as a substitute for 24-hour urine collections. Expressed as the amount excreted per gram of creatinine, urine concentrations in a single-voided sample are often used to estimate 24-hour excretion rates of protein, sodium, potassium, calcium, magnesium, urea, and uric acid. These estimates are predicated on the assumption that daily creatinine excretion equals 1 gm (and that a urine creatinine concentration of 100 mg/dl reflects a 1 Liter 24-hour urine volume). Such estimates are invalid if the serum creatinine concentration is rising or falling. In addition, because creatinine excretion is determined by muscle mass, the assumption that 24-hour urine creatinine excretion equals 1 gm yields a misleading estimate at the extremes of age and body size. In this review we evaluate seven equations for the accuracy of their estimates of urine volume based on urine creatinine concentrations in actual and idealized patients. None of the equations work well in patients who are morbidly obese or in patients with markedly decreased muscle mass. In other patients, estimates based on a reformulation of the Cockroft-Gault equation are reasonably accurate. A recent study based on this relationship found a high strength of correlation between estimated and measured urine output with chronic kidney disease (CKD) studied in the African American Study of Kidney Disease (AASK) trial and for the patients studied in the CKD Optimal Management with Binders and NictomidE (COMBINE) trial. However, the equation systematically underestimated urine output in the AASK trial. Hence, an intercept was added to account for the bias in estimated output. A more rigorous equation, derived from an ambulatory Swiss population, that includes body mass index and models the non-linear accelerated decline in creatinine excretion with age, could potentially be more accurate in overweight and elderly patients. In addition to extremes of body weight and muscle mass, decreased dietary intake or reduced hepatic synthesis of creatine, a precursor of creatinine, or ingestion of creatine supplements will also result in inaccurate estimates. These limitations must be appreciated to rationally use predictive equations to estimate urine volume. If the baseline urine creatinine concentration is determined in a sample of known volume, subsequent urine creatinine concentrations will reveal actual urine output as well as the change in urine output. Given the constraints of the various estimating equations, a single baseline timed collection may be more useful strategy for monitoring urine volume than entering anthropomorphic data into a calculator.

A Formula for the Estimation of 24-Hour Urinary Creatinine Excretion: A Derivation and Validation Study

OBJECTIVE

Twenty-four-hour urinary creatinine (Cr) excretion (24h-uCr) is the basis of Cr clearance and urinary protein-Cr ratio, and it is related to frailty, worsening kidney function, and mortality in patients with chronic kidney disease. Although subjects with lower estimated glomerular filtration rate (eGFR) tend to have lower 24h-uCr, previous formulae for the estimation of 24h-uCr did not include Cr as a predictor.

METHODS

This retrospective study included patients admitted to the Department of Nephrology at our hospital (derivation cohort and validation cohort: patients admitted between April 2016 and March 2020). The prediction formula of 24h-uCr was calculated using a multivariate linear regression model with the bootstrap method. Age, height, weight, sex, Cr, and cystatin C were used as predictors.

RESULTS

The derivation and validation cohorts included 187 and 63 patients, respectively. The characteristics of the derivation and validation cohorts were as follows: age 73 (61-79.5) years and 70 (58.5-79) years; males, 61.5% and 60.3%; eGFR_Cr 27.0 (13.7-48.6) mL/min/1.73 m² and 26.3 (14.0-51.5) mL/min/1.73 m²; and 24-hour urinary protein excretion 0.79 (0.17-2.12) g/day and 1.08 (0.26-2.55) g/day, respectively. Seven prediction formulae were derived. In all models, the Pearson's correlation coefficient was relatively high and statistically significant. However, previous models tended to overestimate the 24h-uCr. Furthermore, the predicted 24h-uCr calculated by the models that do not include Cr as a predictor fluctuates depending on the eGFR_Cr.

CONCLUSION

The best formula for predicting 24h-uCr (mg/day) in a wide range of eGFR populations is a Cr-containing formula: [-9.04 × age (years) + 8.03 × weight (kg) + 0.66 × height (cm) + 188.59 (if male) - 32.11 × Cr (mg/dL) + 779.14].

Addressing the problem of inaccuracy of measured 24-hour urine collections due to incomplete collection

The 24-hour urine collection is widely considered the gold standard for assessing 24-hour excretion of various analytes. Yet, studies show that >30% of collections are incomplete and hence understate the true 24-hour excretion. We previously validated an equation for estimating an individual's 24-hour creatinine excretion, based on weight, sex, race, and age. The present study examines whether routinely correcting measured 24-hour excretion of an analyte using the ratio of estimated to measured 24-hour urine creatinine mitigates the misleadingly low values that result from undercollection. Ninety-nine participants, recruited from an outpatient hypertension clinic, submitted a 24-hour urine collection for measurement of creatinine and sodium excretion. The urine collections were dichotomized into complete (n = 63) and incomplete (n = 36) collections based on previously described criteria for expected 24-hour creatinine excretion. To adjust for possible incompleteness of collections, the measured 24-hour values were then corrected by multiplying them by the ratio of estimated to measured 24-hour urine creatinine. The mean 24-hour creatinine excretion was 1682 mg/d among complete collectors. Among incomplete collectors, the mean was 1211 mg/d before and 1695 mg/d after, adjustment. Similarly, mean 24-hour sodium excretion was 156 mEq/d among complete collectors, whereas among incomplete collectors it was 121 mEq/d before and 171 mEq/d after, adjustment. Undercollection of 24-hour urines is a common problem and results in misleadingly low values for excretion of analytes being measured. Routine adjustment of 24-hour urine values using estimated values for 24-hour creatinine excretion can greatly mitigate artifactually low 24-hour excretion results.

Performance of 24-hour urinary creatinine excretion-estimating equations in relation to measured 24-hour urinary creatinine excretion in hospitalized hypertensive patients

Estimated 24-hour urinary creatinine excretion (24 hrUCr) may be useful for converting spot urine analyte/creatinine ratio into estimated 24-hour urinary excretion of the evaluated analyte, and for verifying completeness of 24-hour urinary collections. We compared various published 24 hrUCr-estimating equations against measured 24 hrUCr in hospitalized hypertensive patients. 24 hrUCr was measured in 293 patients and estimated using eight formulas (CKD-EPI, Cockcroft-Gault, Walser, Goldwasser, Rule, Gerber-Mann, Kawasaki, Tanaka). We used the Pearson correlation coefficient, the Bland-Altman method, and the percentage of estimated 24 hrUCr within 15%, 30% (P30), and 50% of measured 24hUCr to compare estimated and measured 24 hrUCr. Differences between the mean bias by eight formulas were evaluated using the Friedman rank sum test. Overall, the best formulas were CKD-EPI (mean bias 0.002 g/d, P30 86%) and Rule (mean bias 0.022 g/d, P30 89%), although both tended to underestimate 24 hrUCr with higher excretion values. The Gerber-Mann formula and the Asian formulas (Tanaka, Kawasaki) were less precise in our study population but superior in an analysis restricted to subjects with highest measured 24 hrUCr per body weight. We found significant differences between 24 hrUCr-estimating equations in hypertensive patients. In addition, formula performance was critically affected by inclusion criteria based on measured 24 hrUCr per body weight.

A Novel Just-in-Time Contextual Mobile App Intervention to Reduce Sodium Intake in Hypertension: Protocol and Rationale for a Randomized Controlled Trial (LowSalt4Life Trial)

Background

High sodium intake is a significant public health problem in the United States. Interventions that lower sodium intake can decrease blood pressure and improve cardiovascular outcomes. Restaurants and grocery stores are prime targets for intervention with about 77% of all sodium intake in the average US diet coming from processed and restaurant foods.

Objective

This study proposes that a mobile app intervention that promotes low-sodium alternatives at grocery stores and restaurants will reduce dietary intake of sodium and improve confidence following a low-sodium diet in hypertension.

Methods

In this single-center, prospective, open-label study, patients will be randomized to a mobile app or usual care for 8 weeks. We will randomize 50 patients (age>18 years) diagnosed with hypertension and on antihypertensive therapy for at least 3 months in a 1:1 manner stratified by gender. Study subjects will receive the mobile app, LowSalt4Life, or usual dietary advice for 8 weeks. LowSalt4Life provides a multifaceted intervention based on just-in-time contextual tailored messages at grocery stores and restaurants. The primary endpoint is the change in the estimated 24-hour urinary excretion of sodium from spot urine. Secondary outcomes include change in the sodium content of the food frequency questionnaire, confidence in following a low-sodium diet, urine chloride and creatinine dipsticks, and blood pressure.

Results

The project was funded in May 2016 until April 2018. This trial is currently enrolling patients. To date, 26 of the 50 patients needed have been enrolled. Results will be available in the Spring of 2019.

Conclusions

This randomized controlled trial will test the efficacy of just-in-time contextual tailored messages through a novel mobile app 8-week intervention on urinary sodium excretion in patients with hypertension. We will address a critical evidence gap in the care of patients with hypertension. If effective, this intervention could be scaled to assess effects on blood pressure and cardiovascular events in hypertension.

Trial Registration

ClinicalTrials.gov NCT03099343; https://clinicaltrials.gov/ct2/show/NCT03099343 (Archived by WebCite at http://www.webcitation.org/735HNzKlQ)

International Registered Report Identifier (IRRID)

PRR1-10.2196/11282

Validation of daily urinary creatinine excretion measurement by muscle-creatinine equivalence

BACKGROUND

Twenty-four-hour urinary creatinine excretion (24hUCrE) is strongly correlated with skeletal muscle mass (SMM). This study suggests how to exploit the power of the SMM-24hUCrE correlation to assess the accuracy of 24hUCrE measurement.

METHODS

Four hundred and sixty-six men, a subgroup of participants in the 2002-2004 follow-up examination of the Olivetti Heart Study, performed a 24-h urine collection to measure 24hUCrE and underwent bioelectrical impedance analysis to evaluate SMM. Linear regression analysis between 24hUCrE and SMM was used to calculate the muscle-creatinine equivalence and to develop an equation to predict the 24hUCrE depending on SMM. The accuracy of the 24hUCrE measurement was assessed using the change in the SMM-24hUCrE correlation coefficient upon variation in the percentage deviation (%D) between the measured and predicted 24hUCrE.

RESULTS

The calculated muscle-creatinine equivalence was 1 g of 24hUCrE = 22.73 kg of SMM. The %Ds and the corresponding SMM-24hUCrE correlation coefficients were as follows: %D = 3.0, r = .997; %D = 4.7, r = .989; %D = 8.1, r = .963; %D = 10.5, r = .940; %D = 12.6, r = .909; %D = 18.9, r = .825; %D = 25.8, r = .707; %D = 33.5, r = .595; %D = 41.4, r = .453.

CONCLUSION

The increase in %D corresponds to a reduced correlation between muscle mass and creatinine excretion, which indicated a poor performance in the measurement of the 24hUCrE. For studies on single individuals, where small variations in 24hUCrE could be significant, a %D up to 12.6% is suggested; on the other hand, a wider %D interval could be acceptable for population studies.

Clinical utility of spot urine protein-to-creatinine ratio modified by estimated daily creatinine excretion in children

BACKGROUND

The spot urine protein-to-creatinine ratio (UPCR) is widely used to predict 24-h urine protein (24-h UP) excretion. In patients with low daily urine creatinine excretion (UCr), however, the UPCR may overestimate 24-h UP. The aim of this study was to predict 24-h UP using UPCR adjusted by estimated 24-h UCr in children.

METHODS

This study included 442 children whose 24-h UP and spot UPCR were measured concomitantly. Estimated 24-h UCr was calculated using three previously existing equations. We estimated the 24-h UP excretion from UPCR by multiplying the estimated UCr. The results were compared with the measured 24-h UP.

RESULTS

There was a strong correlation between UPCR and 24-h UP (r = 0.801, P < 0.001), and the correlation improved after multiplying the UPCR by the measured UCr (r = 0.847, P < 0.001). Using the estimated UCr rather than the measured UCr, there was high accuracy and strong correlation between the estimated UPCR weighted by the Cockcroft-Gault equation and 24-h UP. Improvement was also observed in the subgroup (proteinuria vs. non-proteinuria) analysis, particularly in the proteinuria group.

CONCLUSIONS

The spot UPCR multiplied by the estimated UCr improved the accuracy of prediction of the 24-h UP in children.

Measurement of muscle mass in sarcopenia: from imaging to biochemical markers

Sarcopenia encompasses the loss of muscle mass and strength/function during aging. Several methods are available for the estimation of muscle or lean body mass. Popular assessment tools include body imaging techniques (e.g., magnetic resonance imaging, computed tomography, dual X-ray absorptiometry, ultrasonography), bioelectric impedance analysis, anthropometric parameters (e.g., calf circumference, mid-arm muscle circumference), and biochemical markers (total or partial body potassium, serum and urinary creatinine, deuterated creatine dilution method). The heterogeneity of the populations to be evaluated as well as the setting in which sarcopenia is investigated impacts the definition of "gold standard" assessment techniques. The aim of this article is to critically review available methods for muscle mass estimation, highlighting strengths and weaknesses of each of them as well as their proposed field of application.

Sex Differences in Renal Proximal Tubular Cell Homeostasis

Studies in human patients and animals have revealed sex-specific differences in susceptibility to renal diseases. Because actions of female sex hormones on normal renal tissue might protect against damage, we searched for potential influences of the female hormone cycle on basic renal functions by studying excretion of urinary marker proteins in healthy human probands. We collected second morning spot urine samples of unmedicated naturally ovulating women, postmenopausal women, and men daily and determined urinary excretion of the renal tubular enzymes fructose-1,6-bisphosphatase and glutathione-S-transferase-α Additionally, we quantified urinary excretion of blood plasma proteins α1-microglobulin, albumin, and IgG. Naturally cycling women showed prominent peaks in the temporal pattern of urinary fructose-1,6-bisphosphatase and glutathione-S-transferase-α release exclusively within 7 days after ovulation or onset of menses. In contrast, postmenopausal women and men showed consistently low levels of urinary fructose-1,6-bisphosphatase excretion over comparable periods. We did not detect changes in urinary α1-microglobulin, albumin, or IgG excretion. Results of this study indicate that proximal tubular tissue architecture, representing a nonreproductive organ-derived epithelium, undergoes periodical adaptations phased by the female reproductive hormone cycle. The temporally delimited higher rate of enzymuria in ovulating women might be a sign of recurring increases of tubular cell turnover that potentially provide enhanced repair capacity and thus, higher resistance to renal damage.