Clinical Mass Spectrometry in the Bioinformatics Era: A Hitchhiker's Guide

Mass spectrometry (MS) is a sensitive, specific and versatile analytical technique in the clinical laboratory that has recently undergone rapid development. From initial use in metabolic profiling, it has matured into applications including clinical toxicology assays, target hormone and metabolite quantitation, and more recently, rapid microbial identification and antimicrobial resistance detection by matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). In this mini-review, we first succinctly outline the basics of clinical mass spectrometry. Examples of hard ionization (electron ionization) and soft ionization (electrospray ionization, MALDI) are presented to demonstrate their clinical applications. Next, a conceptual discourse on mass selection and determination is presented: quadrupole mass filter, time-of-flight mass spectrometer and the Orbitrap; and MS/MS (tandem-in-space, tandem-in-time and data acquisition), illustrated with clinical examples. Current applications in (1) bacterial and fungal identification, antimicrobial susceptibility testing and phylogenetic classification, (2) general unknown urine toxicology screening and expanded new-born metabolic screening and (3) clinical metabolic profiling by gas chromatography are outlined. Finally, major limitations of MS-based techniques, including the technical challenges of matrix effect and isobaric interference; and novel challenges in the post-genomic era, such as protein molecular variants, are critically discussed from the perspective of service laboratories. Computer technology and structural biology have played important roles in the maturation of this field. MS-based techniques have the potential to replace current analytical techniques, and existing expertise and instrument will undergo rapid evolution. Significant automation and adaptation to regulatory requirements are underway. Mass spectrometry is unleashing its potentials in clinical laboratories.

Candidate Reference Measurement Procedure for the Quantification of Total Serum Cortisol with LC-MS/MS

BACKGROUND

Accurate measurement of serum cortisol is required to diagnose and treat adrenal disorders. Although certified reference materials (CRMs) are available to standardize cortisol measurements, External Quality Assessment (EQA) schemes still demonstrate a wide dispersion of results. We present a serum cortisol candidate reference measurement procedure that, through analysis of a Joint Committee for Traceability in Laboratory Medicine–listed panel of higher-order CRMs, provides metrologically traceable results.

METHOD

Isotope-labeled internal standard was added to samples before supported liquid extraction. Extracts were analyzed with LC-MS/MS in positive electrospray ionization mode. Multiple reaction monitoring was used to detect cortisol and its corresponding internal standard transitions. We measured samples in triplicate over 3 days and calculated the mean result.

RESULTS

Mean intra- and interassay imprecision were 1.3% and 1.5%, respectively, for concentrations of 154, 510, and 769 nmol/L. Ionization efficiency studies and structural analog analysis proved the method to be robust against interferences. Through analysis of 34 CRMs (83–764 nmol/L), expanded measurement uncertainty was calculated to be 5% (95% CI). The mean bias between the measured and target CRM concentrations was statistically insignificant at −0.08%.

CONCLUSIONS

The accuracy and low measurement uncertainty of this method qualify it as a CRM procedure. Metrological traceability has been achieved through the analysis of higher-order CRMs. This method could be used to underpin serum cortisol EQA schemes to provide samples with a traceable target value, enabling participating laboratories to determine the accuracy and measurement uncertainty of their assays.

Development of an SI-Traceable HPLC-Isotope Dilution Mass Spectrometry Method To Quantify β-Lactoglobulin in Milk Powders

β-Lactoglobulin (β-LG) is one of the major allergenic proteins in milk. There is an urgent demand for an accurate and traceable method to develop β-LG certified reference material (CRM). In this work, β-LG was enzymatically digested and a specific peptide was chosen for quantitation by isotope-dilution mass spectrometry (IDMS). With amino acid CRMs as standards, the results could be traced to SI unit. By the proposed method, the recovery ranged from 86.0% to 118.3% with CVs <9.0%. The LOD and LOQ were 4.8 × 10^-5 g/g and 1.6 × 10^-4 g/g of β-LG in milk powder, respectively. Ten samples from domestic market were analyzed with CVs <5.6%, and the relative expanded uncertainties ranged from 4.2% to 5.9% (k = 2). With the CRMs, it is expected that the comparability of β-LG quantitation results will be improved among different laboratories.

Metrological sharp shooting for plasma proteins and peptides: The need for reference materials for accurate measurements in clinical proteomics and in vitro diagnostics to generate reliable results

Reliable study results are necessary for the assessment of discoveries, including those from proteomics. Reliable study results are also crucial to increase the likelihood of making a successful choice of biomarker candidates for verification and subsequent validation studies, a current bottleneck for the transition to in vitro diagnostic (IVD). In this respect, a major need for improvement in proteomics appears to be accuracy of measurements, including both trueness and precision of measurement. Standardization and total quality management systems (TQMS) help to provide accurate measurements and reliable results. Reference materials are an essential part of standardization and TQMS in IVD and are crucial to provide metrological correct measurements and for the overall quality assurance process. In this article we give an overview on how reference materials are defined, prepared and what role they play in standardization and TQMS to support the generation of reliable results. We discuss how proteomics can support the establishment of reference materials and biomarker tests for IVD applications, how current reference materials used in IVD may be beneficially applied in proteomics, and we provide considerations on the establishment of reference materials specific for proteomics. For clarity, we solely focus on reference materials related to serum and plasma.

Determination of serum cortisol using isotope dilution-liquid chromatography-mass spectrometry as a candidate reference method

We propose isotope-dilution mass spectrometry as a candidate reference method for determination of serum cortisol. The method uses liquid chromatography-mass spectrometry (LC-MS), interfaced with electrospray ionization, and selective monitoring of the [M + H]+ ions of cortisol and isotopically labeled cortisol. The isotope-dilution-liquid chromatography-mass spectrometry (ID-LC-MS) method simplifies sample-preparation, because samples are processed by simple solvent extraction without further clean-up and derivatization. We studied the time required for complete equilibration of endogenous cortisol and labeled cortisol spiked into serum and found it to be less than 1 h. The repeatability and the reproducibility of the method were evaluated and found to be 0.55% of the measurement value. CRM 192 and 193 from the Bureau Communautaire de Reference were analyzed for verification of the method. The results obtained from the ID-LC-MS method agreed with the certified values. The relative uncertainty of measurement results for samples in the range of a few tens of micrograms per kilogram to several hundred micrograms per kilogram was evaluated and found to be 0.56%. Immunoassay carried out by three independent clinical laboratories produced results more than 15% higher than this ID-LC-MS method, suggesting the presence of bias in the immunoassay methods.

Measuring tobacco smoke exposure: quantifying nicotine/cotinine concentration in biological samples by colorimetry, chromatography and immunoassay methods

Procedures to assess tobacco smoke exposure are reviewed and biomarkers used for determining the smoking status of an individual are compared. Methods used to extract these biomarkers from saliva, urine, and blood and the advantages and disadvantages of the assays are discussed. Finally, the procedures used to measure the levels of cortisol, a stress hormone speculated to be linked to nicotine metabolism, are discussed.

Development of an isotope dilution mass spectrometry assay for HbA1c based on enzyme-cleaved peptide analysis

HbA1c is an index of control in diabetes patients. We report a highly reproducible measurement method for HbA1c based on analysis of the enzyme-cleaved peptide by electrospray ionization mass spectrometry using deuterium-labeled synthetic peptides as internal standards. Intra- and inter-assay coefficients of variation for the novel method ranged from 1.23 to 1.99% for samples with high and low HbA1c. Using this method, we clarified the extent of discrepancies among the indices of diabetes measured by conventional methods and the ESI method for clinical samples including those from patients with Hb variants. High-performance liquid chromatography (HPLC) methods for most samples with variants underestimate the true HbA1c value, although a few variants give a positive error for HbA1c. Immunoassays may also underestimate the values, if the reactivity of the antibody is low against the glycated N-terminal of the variant beta-chains by conformational change. The method proposed here is an important step to establish a candidate definitive method, and is also useful in assessing specific HbA1c test systems using samples containing Hb variants.

Development and Evaluation of a Candidate Reference Method for the Determination of Total Cortisol in Human Serum Using Isotope Dilution Liquid Chromatography/Mass Spectrometry and Liquid Chromatography/Tandem Mass Spectrometry

Cortisol is an important diagnostic marker for the production of steroid hormones, and accurate measurements of serum cortisol are necessary for proper diagnosis of adrenal function. A candidate reference method involving isotope dilution coupled with liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) has been developed and critically evaluated. An isotopically labeled internal standard, cortisol-d(3), was added to serum, followed by equilibration and solid-phase and ethyl acetate extractions to prepare samples for liquid chromatography/mass spectrometry electrospray ionization (LC/MS-ESI) and liquid chromatography/tandem mass spectrometry electrospray ionization (LC/MS/MS-ESI) analyses. (M + H)(+) ions at m/z 363 and 366 for cortisol and its labeled internal standard were monitored for LC/MS. The transitions of (M + H)(+) --> [(M + H)(+) - 2H(2)O] at m/z 363 --> 327 and 366 --> 330 were monitored for LC/MS/MS. The accuracy of the measurement was evaluated by a comparison of results of this candidate reference method on lyophilized human serum reference materials for cortisol [Certified Reference Materials 192 and 193] with the certified values determined by gas chromatography/mass spectrometry reference methods and by a recovery study for the added cortisol. The results of this method for total cortisol agreed with the certified values within 1.1%. The recovery of the added cortisol ranged from 99.8% to 101.0%. This method was applied to the determination of cortisol in samples of frozen serum pools. Excellent precision was obtained with within-set CVs of 0.3%-1.5% and between-set CVs of 0.04%-0.4% for both LC/MS and LC/MS/MS analyses. The correlation coefficients of all linear regression lines ranged from 0.998 to 1.000. The detection limits (at a signal-to-noise ratio of approximately 3-5) were 10 and 15 pg for LC/MS and LC/MS/MS, respectively. This method, which demonstrates good accuracy and precision, and is free from interferences from structural analogues, qualifies as a candidate reference method and can be used as an alternative reference method to provide an accuracy base to which the routine methods can be compared.

Determination of serum cortisol by isotope-dilution liquid-chromatography electrospray ionization tandem mass spectrometry with on-line extraction

A liquid chromatographic-mass spectrometric method for the determination of cortisol in serum using atmospheric pressure electrospray ionization and tandem mass spectrometry is described. During sample preparation, 150 microl of serum were deproteinized with methanol/zinc sulfate followed by on-line solid phase extraction employing column switching. Tri-deuterated cortisol was used as the internal standard. The following transitions were monitored: cortisol, 363>309 m/z; d3-cortisol, 366>312 m/z. The total run-time was 5 minutes. The method proved linear (0-500 microg/l; r=0.999), precise (total coefficient of variation between 5.0% and 3.2% at a mean cortisol concentration of 15.1 microg/l and 269 microg/l, respectively; n=16) and specific with regard to relevant endogenous and exogenous steroids.

Validation of Accuracy-based Amino Acid Reference Materials in Dried-Blood Spots by Tandem Mass Spectrometry for Newborn Screening Assays

Background: Advances in technology and the earlier release of newborns from hospitals have pressed the demand for accurate calibration and improved interlaboratory performance for newborn screening tests. As a first step toward standardization of newborn screening aminoacidopathy tests, we have produced six-pool sets of multianalyte dried-blood-spot amino acid reference materials (AARMs) containing predetermined quantities of five amino acids. We describe here the production of the AARMs, validation of their amino acid contents, and characterization of their homogeneity and their stability in storage.

Methods: To each of six portions of a pool of washed erythrocytes suspended in serum we added Phe (0–200 mg/L), Leu (0–200 mg/L), Met (0–125 mg/L), Tyr (0–125 mg/L), and Val (0–125 mg/L). Six-pool sets (1300) were prepared, dried, and packaged. We used isotope-dilution mass spectrometry to estimate the endogenous amino acid concentrations of the AARMs and validate their final amino acid concentrations. We used additional tandem mass spectrometry analyses to examine the homogeneity of amino acid distribution in each AARM, and HPLC analyses to evaluate the stability of the amino acid contents of the AARMs.

Results: The absolute mean biases across the analytic range for five amino acids were 2.8–9.4%. One-way ANOVAs of the homogeneity results predicted no statistically significant differences in amino acid concentrations within the blood spots or within the pools (P &gt;0.05). Regression slopes (0 ± 0.01) for amino acid concentrations vs storage times and their P values (&gt;0.05) showed no evidence of amino acid degradation at ambient temperatures, 4 °C, or −20 °C during the intervals tested.

Conclusion: The validation, homogeneity, and stability of these blood spots support their use as a candidate national reference material for calibration of assays that measure amino acids in dried-blood spots.

Clinical applications of gas chromatography and gas chromatography-mass spectrometry of steroids

This review article underlines the importance of gas chromatography-mass spectrometry (GC-MS) for determination of steroids in man. The use of steroids labelled with stable isotopes as internal standard and subsequent analysis by GC-MS yields up to now the only reliable measurement of steroids in serum. Isotope dilution GC-MS is the reference method for evaluation of routine analysis of serum steroid hormones. GC-MS is an important tool for detection of steroid hormone doping and combined with a combustion furnace and an isotope ratio mass spectrometer the misuse of testosterone by athletes can be discovered. Finally the so called urinary steroid profile by GC and GC-MS is the method of choice for detection of steroid metabolites in health and disease.

A reference system for cortisol

OBJECTIVES

The International Federation of Clinical Chemistry (IFCC) initiated a pilot study, with cortisol as an example, that aims to implement the concept of standardization of hapten immunoprocedures on the basis of metrological traceability. In fact, this standardization concept comes down to correct calibration of measurement procedures, so that measurement results for patient samples can be traced back to the metrologically highest reference of a result, i.e., an SI unit as embodied in the primary reference material. In consequence, demonstration of standardization of a test system on the basis of traceability requires evaluation of measurement results pertaining to patient samples for accuracy. Such an evaluation shall be done by a correlation study of the routine test system with an accuracy-based reference measurement procedure.

DESIGN AND METHODS

The IFCC project will select a panel of single donation human serum samples, and assign them with values for cortisol by measurement with at least two isotope dilution-gas chromatography/mass spectrometry reference methods. Limited amounts of the panel will be distributed to manufacturers, with the object to measure the samples with their calibrated immunoprocedures. The patient correlation studies between the routine and the reference methods will then be interpreted in terms of specificity and accuracy. It will also be investigated whether the performed comparison can be used as a basis for re-calibration. From the experience gained through this pilot study for cortisol, IFCC will formulate recommendations and guidance to manufacturers on how to perform and reliably interpret patient correlation studies. In this way, it is to be expected that the project might form a basis for general implementation of the concept of standardization of hapten immunoassays on the basis of metrological traceability.

Measurement of urinary free cortisol by stable isotope dilution mass spectrometry using a new cortisol derivative

The reaction of the bismethylenedioxy derivative of cortisol (cortisol-BMD) with heptafluorobutyric anhydride to give the corresponding 3,5-dienol heptafluorobutyrate (cortisol-BMD-HFB) has been shown to proceed with dehydration. Acid-promoted dehydration of either cortisol-BMD or cortisol-BMD-HFB, or concurrent dehydration of both, is the proposed reaction mechanism leading to a trienol heptafluorobutyrate, whose chromatographic properties and mass spectral data are consistent with the additional double bond in the C9-C11 position. Forming the 3,5-dienol heptafluorobutyrate of cortisol-BMD in benzene rather than acetone gave a compound whose chromatographic properties and mass spectral data were different to that of the 3,5,9(11)-trienol heptafluorobutyrate but consistent with that of a cortisol-BMD-HFB. The mass fragmentometry of this new cortisol derivative was found to be more suited to the technique of isotope dilution mass spectrometry than the 3,5,9(11)-trienol heptafluorobutyrate, and thus was applied to our intended goal of measuring urinary free cortisol by gas chromatography-mass spectrometry. An efficient and convenient solid-phase extraction technique is employed in our assay to isolate cortisol from 5 ml of urine.

Increasing thermospray response for cortisol by derivatization

The 21-hydroxyl group of cortisol was selectively acetylated under mild conditions without affecting the more sterically hindered 11 beta- and 17 alpha-hydroxyl groups. The reaction was performed with a mixture of acetic anhydride and triethylamine in acetonitrile and was complete in less than 15 min at room temperature. The thermospray mass spectrum of cortisol 21-acetate showed minimal fragmentation with the [M + H]+ ion as the base peak. High sensitivity was achieved for acetylated cortisol during selected ion monitoring, the signal-to-noise ratio being increased by a factor of about 4, compared to underivatized cortisol. The limit of detection of cortisol 21-acetate was estimated at 0.24 pmol injected, making thermospray liquid chromatography-mass spectrometry competitive with gas chromatography-mass spectrometry for the determination of cortisol in biological fluids.

Reference methods as a basis for accurate measuring systems

Reference methods, as part of a reference system, are essential for improving the accuracy of routine methods. They can help to establish worldwide compatibility of clinical chemical measurements and they provide an objective basis for quality control measures. The subject of analytical errors is treated in detail, then the concept of reference methods is developed. This review emphasizes the fundamental metrological aspects of reference methods and reference measurement technology. An overview is given of the most important analytical principles used for reference methods in clinical chemistry. The particular problems of reference methods for the determination of the catalytic activity concentration of enzymes are outside the scope of this review.

Cortisol production rates measured by liquid chromatography/mass spectrometry

Cortisol production rates (FPRs) in physiologic and pathologic states in humans have been investigated over the past 30 years. However, there has been conflicting evidence concerning the validity of the currently accepted value of FPRs in humans (12 to 15 mg/m2/d) as determined by radiotracer methodology. The present study reviews previous methods proposed for the measurement of FPRs in humans and discusses the applications of the first method for the direct determination of 24-hour plasma FPRs during continuous administration of a stable isotope, using a thermospray high-pressure liquid chromatography-mass spectrometry technique. The technique is fast, sensitive, and, unlike gas chromatography-mass spectrometry methods, does not require derivatization, allowing on-line detection and quantification of plasma cortisol after a simple extraction procedure. The results of determination of plasma FPRs by stable tracer/mass spectrometry are directly in units of mass/time and, unlike radiotracer methods, are independent of any determination of volume of distribution or cortisol concentration. Our methodology offers distinct advantages over radiotracer techniques in simplicity and reliability since only single measurements of isotope ratios are required. The technique was validated in adrenalectomized patients. Circadian variations in daily FRPs were observed in normal volunteers, and, to date, results suggest a lower FRP in normal children and adults than previously believed.

Determination of cortisol in human plasma by stable isotope dilution mass spectrometry

Cortisol selectively labelled with 2H at the 19-methyl and the C-1 position (cortisol-d5) was synthesized and used as an internal standard in stable isotope dilution mass spectrometry in order to determine cortisol in human plasma. A capillary gas chromatographic/mass spectrometric method provided a sensitive and reliable technique with good accuracy, precision and reproducibility without complex corrections for contributions by using cortisol-d5 as an internal standard. For calculation of plasma cortisol, peak areas were measured by selected ion monitoring on the characteristic fragment ions of the dimethoxime tri(trimethylsilyl) derivatives of cortisol and cortisol-d5 (m/z 605 and 610, respectively). The sensitivity of the gas chromatographic/mass spectrometric assay was 1.02 ng per injection, with a signal-to-noise ratio of about 6. The inter- and intra-assay coefficients of variation for plasma sample were 3.07% and 1.77%, respectively.

Stable isotope dilution method using thermospray liquid chromatography/mass spectrometry for quantification of daily cortisol production in humans

A thermospray high-performance liquid chromatography/mass spectrometry method for the separation and quantification of cortisol in human serum has been developed. The technique does not require derivatization, allows for both qualitative and quantitative determinations, provides increased specificity not available from conventional high-performance liquid chromatography, and has a detection limit of 5 pmol on-column. This isotope dilution mass spectrometry method, using d3-cortisol as an internal standard, allows precise determinations even at low isotopic ratios (2%-5 mole%). Evidence that this technique can be applied to the quantification of serum cortisol and to the determination of daily cortisol production in humans is presented.

The quantification of synthetic corticosteroids using isotope dilution gas chromatography negative chemical ionization mass spectrometry

Prednisolone, dexamethasone and betamethasone were labelled with deuterium via a simple synthetic procedure and used as internal standards in the gas chromatographic/mass spectrometric analysis of the corresponding undeuterated compounds. The mass spectrometer was used in the negative chemical ionization mode, which gave fragmentation of the methoxime trimethylsilyl ether derivatives favourable for their quantification. The method was applicable to the quantification of synthetic corticosteroids contained in human aqueous humour in the 0.1-10-ng range.

Determination of cortisol in human plasma by capillary gas chromatography-mass spectrometry using [2H5] cortisol as an internal standard

A capillary gas chromatographic-mass spectrometric method for the determination of cortisol in human plasma using cortisol M + 5 as an internal standard is described. For calculation of plasma cortisol, peak areas were measured by selected-ion monitoring of the characteristic fragment ions of the dimethoxime-tri (trimethylsilyl) derivatives of cortisol and cortisol M + 5 (m/z 605 and 610, respectively). The inter- and intra-assay coefficients of variation for plasma sample were 3.07 and 1.77%, respectively. The method needed no complex corrections for contributions and provides a sensitive and reliable technique with good accuracy, precision and reproducibility.

High-performance liquid chromatography of steroid hormones

Although a considerable amount of work has been carried out in the last ten years in developing methods for the separation of steroids by HPLC, it is still not widespread for the reasons discussed above. There is however no doubt that further developments in HPLC technology, in increasing sensitivity and/or specificity of detection systems, perhaps with microbore columns, may lead to an increase in the use of this powerful analytical procedure as an additional separation method to improve specificity of assay. Solution of the problem of simple interfacing of HPLC systems with mass spectrometers (discussed in another chapter by Games) should further increase the application of HPLC. HPLC is of particular value in providing a means of separating unstable compounds prior to assay by relatively nonspecific quantitation methods. Most steroids do not fall into this category, but the steroid vitamin D and its metabolites do and HPLC has proved in this area to be invaluable (see chapter by Jones & DeLuca). There are a multiplicity of different HPLC systems for the separation of steroids, varying in column type (and manufacturer), solvent composition and method of elution, temperature of elution, etc., and only a few attempts have been made to rationalise these data. It would therefore seem that a fruitful area of future study would be the investigation of computerised systems for the selection and optimisation of HPLC systems for particular steroid separations.

Quantitative analysis of adenosine: statistical comparison of radioimmunoassay and gas chromatography-mass spectrometry-selected ion monitoring methods

A new method for quantitating adenosine concentration by capillary gas chromatography-mass spectrometry-selected ion monitoring (GC-MS-SIM) has been developed and used as a reference method for evaluating a newly developed radioimmunoassay (RIA) for adenosine. Details of the GC-MS-SIM method are presented, along with the comparative results and uncertainties of both methods. General considerations in the statistical analysis of method comparison data are discussed with particular reference to studies using quantitative mass spectrometry as the standard method; the adenosine methods are used as specific examples in this discussion. Simultaneous estimation of the y-intercept and slope of the least squares regression line relating the results of the two methods using the 95% joint confidence ellipse demonstrated the absence of either constant or proportional error between the two methods. The relatively small uncertainty in the GC-MS-SIM measurements had no significant effect on the linear regression. Random error between the two methods was detected, and was estimated by the coefficient of variation in the RIA data as ten percent of the RIA value.

Measurement of testosterone and its 5-alfa-reduced metabolites in human prostatic tissue using isotope dilution mass spectrometry

5 alpha-Dihydrotestosterone (DHT) has been widely measured in human prostatic tissue using RIA since it is found to be involved in pathogenesis of human prostatic hyperplasia (BHP) and to be the best index for the follow-up of patients affected by prostatic cancer under endocrine treatment. A GC/MS method for the simultaneous determination of testosterone (T), dihydrotestosterone (DHT), 5 alpha-androstan-3 alpha,17 beta-diol (3 alpha-diol) and 5 alpha-androstan-3 beta-17 beta-diol (3 beta-diol) in prostatic tissue was developed based on the isotopic dilution technique. Trideuterated internal standards of each compound were previously synthesized in our laboratory. After previous extraction and purification on Sep-Pak C-18 cartridges and Lipidex DEAP columns, T and its metabolites were measured as heptafluorobutyric esters (HFB). Quantitative analysis was performed on a VG 7070 EQ mass spectrometer equipped with a fused silica capillary column using the Selected Ion Monitoring technique. Steroid values (means +/- SD; ng/g tissue) found in 9 human hypertrophic prostates were: T, 0.71 +/- 0.43; DHT, 4.46 +/- 1.41; 3 alpha-diol, 0.34 +/- 0.23; 3 beta-diol, 0.14 +/- 0.32.

Isotope dilution analysis using chromatographic separation of isotopic forms of the compound to be measured

Using progesterone, testosterone, androstenedione, 11-oxoprogesterone and 11 beta-hydroxyprogesterone as models, a new form of isotope dilution assay has been developed. A known mass of deuterium-labelled steroid is added to the serum sample. High-performance liquid chromatography is used to separate endogenous steroid from its deuterium-labelled form. After separation, the two forms of the analyte are quantitated using conventional methods: radioimmunoassay, enzyme-linked immunoassay and, where the concentrations are high enough, ultraviolet light absorption. The ratio of the amounts of the two forms of the analyte is used to calculate the amount of unlabelled material in the original sample. The assay principle is quite general. A variety of high resolution methods are available to separate isotopic analogues of the same compound. A number of detection methods can be used to quantitate the separated isotopic forms. Extension of this principle to other fields of interest in bio-medicine is discussed briefly.