Pitfalls of immunoassay and sample for IGF-I: comparison of different assay methodologies using various fresh and stored serum samples

Challenges of insulin-like growth factor-1 testing

Insulin-like growth factor 1 (IGF-1), primarily synthesized in the liver, was initially discovered due to its capacity to replicate the metabolic effects of insulin. Subsequently, it emerged as a key regulator of the actions of growth hormone (GH), managing critical processes like cell proliferation, differentiation, and apoptosis. Notably, IGF-1 displays a longer half-life compared to GH, making it less susceptible to factors that may affect GH concentrations. Consequently, the measurement of IGF-1 proves to be more specific and sensitive when diagnosing conditions such as acromegaly or GH deficiency. The recognition of the existence of IGFBPs and their potential to interfere with IGF-1 immunoassays urged the implementation of various techniques to moderate this issue and provide accurate IGF-1 results. Additionally, in response to the limitations associated with IGF-1 immunoassays and the occurrence of discordant IGF-1 results, modern mass spectrometric methods were developed to facilitate the quantification of IGF-1 levels. Taking advantage of their ability to minimize the interference caused by IGF-1 variants, mass spectrometric methods offer the capacity to deliver robust, reliable, and accurate IGF-1 results, relying on the precision of mass measurements. This also enables the potential detection of pathogenic mutations through protein sequence analysis. However, despite the analytical challenges, the discordance in IGF-1 reference intervals can be attributed to a multitude of factors, potentially leading to distinct interpretations of results. The establishment of reference intervals for each assay is a demanding task, and it requires nationwide multicenter collaboration among laboratorians, clinicians, and assay manufacturers to achieve this common goal in a cost-effective and resource-efficient manner. In this comprehensive review, we examine the challenges associated with the standardization of IGF-1 measurement methods, the minimization of pre-analytical factors, and the harmonization of reference intervals. Particular emphasis will be placed on the development of IGF-1 measurement techniques using "top-down" or "bottom-up" mass spectrometric methods.

Insulin-Like Growth Factor-1 (IGF-1) and Its Monitoring in Medical Diagnostic and in Sports

Insulin-like growth factor-1 (IGF-1) is the principal mediator of growth hormone (GH), plays a crucial role in promoting cell growth and differentiation in childhood and continues to have an anabolic effect in adults. IGF-1 is part of a wide network of growth factors, receptors and binding proteins involved in mediating cellular proliferation, differentiation and apoptosis. Bioavailability of IGF-1 is affected by insulin-like growth factor binding proteins (IGFBPs) which bind IGF-1 in circulation with an affinity equal to or greater than that of the IGF-1 receptor (IGF-1R). The six IGFBPs serve as carrier proteins and bind approximately 98% of all circulating IGF-1. Other proteins known to bind IGF-1 include ten IGFBP-related proteins (IGFBP-rPs), albeit with lower affinities than the IGFBPs. IGF-1 expression levels vary in a number of clinical conditions suggesting it has the potential to provide crucial information as to the state of an individual’s health. IGF-1 is also a popular doping agent in sport and has featured in many high-profile doping cases in recent years. However, the existence of IGFBPs significantly reduces the levels of immunoreactive IGF-1 in samples, requiring multiple pre-treatment steps that reduce reproducibility and complicates interpretation of IGF-1 assay results. Here we provide an overview of the IGF network of growth factors, their receptors and the entirety of the extended family of IGFBPs, IGFBP-rPs, E peptides as well as recombinant IGF-1 and their derivatives. We also discuss issues related to the detection and quantification of bioavailable IGF-1.

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.

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’.

The current status of IGF-I assays--a 2009 update

For almost three decades, the measurement of circulating IGF-I has constituted a highly important biochemical tool in the management of GH disorders. In fact, in acromegaly the importance of circulating IGF-I has increased following the introduction of the GH receptor antagonist pegvisomant, as the use of this drug makes it impossible to use circulating GH as a monitor of disease activity. In addition, determination of circulating IGF-I constitutes a valuable scientific tool in various research areas, from epidemiological investigations through clinical trials and experimental studies. The multiple facets of IGF-I physiology and patho-physiology may explain why numerous endocrine laboratories have invested in IGF-I assays, by means of either in-house assays or commercial kits. However, despite its widespread use, the measurement of IGF-I is by no means trivial. On the contrary, the pronounced binding of IGF-I to the high-affinity IGF-binding proteins (IGFBPs) constitutes a notorious source of error, which has necessitated the development of methods that more or less successfully circumvent interference from the IGFBPs. Furthermore, there are some unsolved issues with the international standardization of the different IGF-I assays and there is no consensus regarding the procedures used when collecting and storing samples for measurement of circulating IGF-I. The aim of this review is to discuss the current state of the art of IGF-I immunoassays and to present the current analytical problems with IGF-I measurements. Finally, we would like to suggest an agenda that may be used when trying to produce internationally accepted uniform requirements for future IGF-I assays.

Profile of mecasermin for the long-term treatment of growth failure in children and adolescents with severe primary IGF-1 deficiency

Growth hormone insensitivity syndrome (GHI) or insulin-like growth factor-1 (IGF-1) deficiency (IGFD) is characterized by deficit of IGF-1 production due to alteration of response of growth hormone (GH) receptor to GH. This syndrome is due to mutation of GH receptor or IGF-1 gene and patients affected showed no response to GH therapy. The only treatment is recombinant IGF-1 (mecasermin), which has been available since 1986, but approved in the United States by the US Food and Drug Administration only in 2005 and in Europe by the European Medicines Agency in 2007. To date, few studies are available on long-term treatment with mecasermin in IGFD patients and some of them have a very small number of subjects. In this review we discuss briefly clinical features of severe primary IGFD, laboratory findings, and indications for treatment. Results of long-term therapy with rhIGF1 (mecasermin) in patients affected by severe primary IGFD and possible side effects are explained.

Laboratory issues in the implementation of the marker method

Two important biomarkers for the identification of growth hormone or IGF-I administration are IGF-I and P-III-P. These substances are determined in plasma or preferably in serum. There are a number of assays on the market for IGF-I but only two for P-III-P. The principles behind these assays and the choice of assays for doping control purposes are discussed. The future possibility of quantification by mass spectrometry is also briefly discussed.

Circulating insulin-like growth factor peptides and prostate cancer risk: a systematic review and meta-analysis

Insulin-like growth factors (IGF-I, IGF-II) and their binding proteins (IGFBP-1-6) play a key role in cell proliferation, differentiation and apoptosis, suggesting possible involvement in carcinogenesis. Several epidemiological studies show associations of IGFs with prostate cancer. We searched the published literature for all studies relating levels of IGFs or IGFBPs with prostate cancer. We performed random effects meta-analysis to calculate summary odds ratios. The number of studies (prostate cancer cases) included in each meta-analysis were 42 (7,481) IGF-I; 10 (923) IGF-II; 3 (485) IGFBP-1; 5 (577) IGFBP-2; 29 (6,541) IGFBP-3 and 11 (3,545) IGF-1:IGFBP-3 ratio. The pooled odds ratios (95% confidence intervals) per standard deviation increase in peptide were: IGF-I, OR = 1.21 (1.07, 1.36); IGF-II, OR = 1.17 (0.93, 1.47); IGFBP-1, OR = 1.21 (0.62, 2.33); IGFBP-2, OR = 1.18 (0.90, 1.54); IGFBP-3, OR = 0.88 (0.79, 0.98); IGFI:IGFBP-3 ratio, OR = 1.10 (0.97, 1.24). For all exposures, there was substantial heterogeneity (all I(2) > 75%), partly explained by study design: the magnitude of associations was smaller in prospective vs. retrospective studies, and for IGFBP-3, the inverse association with prostate cancer risk was seen in retrospective but not prospective studies. There was weak evidence that associations of IGF-I and IGFBP-3 with prostate cancer were stronger for advanced disease. Our meta-analysis confirms that raised circulating lGF-I is positively associated with prostate cancer risk. Associations between IGFBP-3 and prostate cancer were inconsistent, and there was little evidence for a role of IGF-II, IGFBP-1 or IGFBP-2 in prostate cancer risk.

Idiopathic short stature: conundrums of definition and treatment

Children with idiopathic short stature (ISS) are statistically defined by height SDS < −2 for their bone age and should be distinguished from children with familial short stature for whom height SDS corresponds to mean parental SDS and from the most common explanation for short stature referred to pediatric endocrinologists, constitutional delay in growth and maturation (CDGM), in which there is normal height for bone age and predicted normal adult stature. Low IGF-I levels reported in ISS may be the result of subtle undernutrition or reference to standards appropriate for chronologic age but not osseous maturation in CDGM inappropriately labeled as ISS. While growth hormone (GH) treatment of ISS may add 4-5 cm to adult height, meta-analysis indicates that there is no documented evidence that such treatment improves health related quality of life or psychological adaptation. Thus, the estimated cost of US$52 000/inch gained is difficult to justify. Absence of data regarding efficacy of the use of IGF-I for treatment of ISS has been noted in a recent consensus statement from the North American and European pediatric endocrinology societies. This report further emphasizes the importance of discouraging the expectation that taller stature from GH treatment will improve quality of life.

Influence of ethnicity on IGF-I and procollagen III peptide (P-III-P) in elite athletes and its effect on the ability to detect GH abuse

CONTEXT

A method based on the two GH dependent markers, IGF-I and procollagen III peptide (P-III-P) has been proposed to detect exogenously administered GH. As previous studies involved predominantly white European elite athletes, it is necessary to validate the method in other ethnic groups.

OBJECTIVE

To examine serum IGF-I and P-III-P in elite athletes of different ethnicities within 2 h of competing at national or international events.

DESIGN

Cross-sectional observational study.

SETTING

National and International sporting events.

SUBJECTS

1085 elite athletes of different ethnicities.

INTERVENTION

Serum IGF-I and P-III-P were measured and GH-2000 discriminant function score was calculated. Effect of ethnicity was assessed.

RESULTS

In men, IGF-I was 21.7 +/- 2.6% lower in Afro-Caribbeans than white Europeans (P < 0.0001) but there were no differences between other ethnic groups. In women, IGF-I was 14.2 +/- 5.1% lower in Afro-Caribbeans (P = 0.005) and 15.6 +/- 7.0% higher in Orientals (P = 0.02) compared with white Europeans. P-III-P was 15.2 +/- 3.5%, 26.6 +/- 6.6% and 19.3 +/- 5.8% lower in Afro-Caribbean (P < 0.0001), Indo-Asian (P < 0.0001) and Oriental men (P = 0.001), respectively, compared with white European men. In women, P-III-P was 15.7 +/- 4.7% lower in Afro-Caribbeans compared to white Europeans (P =0.0009) but there were no differences between other ethnicities. Despite these differences, most observations were below the upper 99% prediction limits derived from white European athletes. All GH-2000 scores lay below the cut-off limit proposed for doping.

CONCLUSIONS

The GH-2000 detection method based on IGF-I and P-III-P would be valid in all ethnic groups.

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.

Intact and total insulin-like growth factor-binding protein-3 (IGFBP-3) levels in relation to breast cancer risk factors: a cross-sectional study

Introduction

Levels of insulin-like growth factor (IGF)-I and its main binding protein (IGFBP-3) have been associated with breast cancer risk among premenopausal women. However, associations of IGFBP-3 levels with breast cancer risk have been inconsistent, possibly due to the different predominant forms of circulating IGFBP-3 (intact versus fragmented) that were measured in these studies. Here, we examine the association of breast cancer risk factors with intact and total IGFBP-3 levels.

Methods

This cross-sectional study includes 737 premenopausal women recruited at screening mammography. Plasma intact and total IGFBP-3 and IGF-I levels were measured by enzyme-linked immunosorbent assay methods. Percent and absolute breast density were estimated using a computer-assisted method. The associations were evaluated using generalized linear models and Pearson (r) or Spearman (r_s) partial correlation coefficients.

Results

Means ± standard deviations of intact and total IGFBP-3 levels (ng/mL) were 1,044 ± 234 and 4,806 ± 910, respectively. Intact and total IGFBP-3 levels were correlated with age and smoking. Levels of intact IGFBP-3 were negatively correlated with waist-to-hip ratio (WHR) (r = -0.128; P = 0.0005), parity (r_s = -0.078; P = 0.04), and alcohol intake (r = -0.137; P = 0.0002) and positively correlated with energy intake (r = 0.075; P = 0.04). In contrast, total IGFBP-3 levels were positively correlated with WHR (r = 0.115; P = 0.002), parity (r_s = 0.089; P = 0.02), body mass index (BMI) (r = 0.115; P = 0.002), physical activity (r = 0.118; P = 0.002), and IGF-I levels (r = 0.588; P < 0.0001) and negatively correlated with percent or absolute breast density (r = -0.095; P = 0.01 and r = -0.075; P = 0.04, respectively).

Conclusion

Our data show that associations of some breast cancer risk factors with intact levels of IGFBP-3 are different from those with total (intact and fragmented) IGFBP-3 levels. These findings suggest that different molecular forms of IGFBP-3 may bear different relations to premenopausal breast cancer risk.

Insulin-like growth factor-I (rhIGF-I) therapy of short stature

The United States Food and Drug Administration (FDA) approved the use of subcutaneously injected rhIGF-I in late 2005 for treatment of children with severe short stature from growth hormone (GH) insensitivity due to genetic defects in the GH receptor or postreceptor mechanisms or from the development of GH inactivating antibodies. The approval was based on 15 years experience treating these rare conditions with rhIGF-I. Because of the very small numbers of children with these conditions, there has been an effort to justify and promote broader use for rhIGF-I. Attempts to identify GH unresponsiveness in children with idiopathic short stature (ISS) have yielded only a handful of patients with rare genetic disorders. IGF-I treatment for unequivocal GH insensitivity improves but does not correct growth failure, in contrast to the typical experience with GH replacement of GH deficiency. This emphasizes the importance of direct effects of GH at the growth plate, including the stimulation of maturation of cartilage precursor cells and local production of IGF-I, effects that cannot be duplicated by exogenous administration of rhIGF-I. Adverse effects testify to the more than adequate delivery of administered rhIGF-I to other tissues; these include lymphoid hyperplasia, coarsening of the facies, and increased percentage body fat. The absence of convincing evidence of GH insensitivity in a substantial number of children with ISS, the limited ability of endocrine IGF-I to restore normal growth in those with unequivocal GH unresponsiveness, the suppression of endogenous GH (and thereby, local GH effects on growth) that occurs with IGF-I administration, the risk profile, and the absence of data on efficacy in other than proven severe GH insensitivity, led the Drug and Therapeutics Committee of the Lawson Wilkins Pediatric Endocrine Society to conclude that rhIGF-I use is only justified in conditions approved by the FDA and that other growth promotional use should only be investigational. Nonetheless, substantial numbers of children are being treated with rhIGF-I off-label, exuberant estimates of potentially eligible patients are projected, and several uncontrolled clinical trials have been undertaken which are not based on sound preliminary data or established growth principles, and a single four-arm study begun comparing monotherapy with rhGH to combination rhGH with three dosages of rhIGF-I as a single daily injection, a means of administration of rhIGF-I that has not been tested.

Serum levels of insulin-like growth factor-I and insulin-like growth factor-I binding protein-3: quality control for studies of stored serum

The insulin-like growth factor (IGF) axis, particularly IGF-I and IGF binding protein-3 (IGFBP-3), has been the subject of much attention because of its role in juvenile growth and their association with cancers at several sites. However, epidemiologic studies of IGF-I and IGFBP-3 have had mixed results and several authors have speculated that quality control (QC), sample storage history, and other methodologic concerns could play a role in this heterogeneity. This article documents the results of storage history and QC efforts for a study of IGF-I and IGFBP-3 in 6,226 serum samples from the National Health and Nutrition Examination Survey III (NHANES III). The study was carried out on site at Diagnostic Systems Laboratories in Webster, Texas, using the IGF-I ELISA (DSL 10-5600) and the IGFBP-3 immunoradiometric assay (DSL 6600). A run-in study of assay performance suggested that plates, days, and weeks significantly affected the variance of both assays. Analysis of samples with different storage histories also indicated strong effects of storage history. Serum samples disbursed to laboratories for measurement of diverse analytes and then returned for storage showed reductions in serum IGF-I level averaging 43% and reductions in IGFBP-3 of 25% compared with samples shipped immediately to the repository for long-term storage at -80 degrees C. Therefore, the main study was carried out using samples that had been shipped directly to the National Center for Health Statistics/NHANES collection center for storage. Laboratory analyses of NHANES III and QC samples were carried out over approximately 10 months. QC was monitored through repeated testing of blood samples from six individuals, with two individuals tested twice on each plate. Assay performance was stable over the entire study and coefficients of variation averaged 2% to 3% within plates and approximately 14% for IGF-I and approximately 11.5% for IGFBP-3 over the entire study. Coefficients of variation varied significantly among individual QC subjects, ranging from 12.3% to 17.6% for IGF-I and 8.9% to 12.8% for IGFBP-3. Based on Levy-Jennings plots, approximately 5% of the plates used for IGF-I in the main study were out of compliance. Finally, location on a plate had small but significant effects on IGF-I level. Together, these results highlight the need for care in large studies of putative biomarkers for cancer risk and illustrate some probable sources of heterogeneity in past epidemiologic studies of the IGF axis and cancer.

Effect of long-term storage on hormone measurements in samples from pregnant women: the experience of the Finnish Maternity Cohort

Validity of biobank studies on hormone associated cancers depend on the extent the sample preservation is affecting the hormone measurements. We investigated the effect of long-term storage (up to 22 years) on immunoassay measurements of three groups of hormones and associated proteins: sex-steroids [estradiol, progesterone, testosterone, dihydroepiandrosterone sulphate (DHEAS), sex hormone-binding globulin (SHBG)], pregnancy-specific hormones [human chorionic gonadotropin (hCG), placental growth hormone (pGH), alpha-fetoprotein (AFP)], and insulin-like growth factor (IGF) family hormones exploiting the world largest serum bank, the Finnish Maternity Cohort (FMC). Hormones of interest were analyzed in a random sample of 154 Finnish women in the median age (29.5 years, range 25 to 34 years) of their first pregnancy with serum samples drawn during the first trimester. All hormone measurements were performed using commercial enzyme-linked- or radio-immunoassays. Storage time did not correlate with serum levels of testosterone, DHEAS, hCG, pGH and total IGFBP-1. It had a weak or moderate negative correlation with serum levels of progesterone (Spearman's ranked correlation coefficient (r(s))=- 0.36), IGF-I (r(s)=-0.23) and IGF binding protein (BP)-3 (r(s)=-0.38), and weak positive correlation with estradiol (r(s)=0.23), SHBG (r(s)=0.16), AFP (r(s)=0.20) and non-phosphorylated IGF binding protein (BP)-1 (r(s)=0.27). The variation of all hormone levels studied followed the kinetics reported for early pregnancy. Bench-lag time (the time between sample collection and freezing for storage) did not materially affect the serum hormone levels. In conclusion, the stored FMC serum samples can be used to study hormone-disease associations, but close matching for storage time and gestational day are necessary design components of all related biobank studies.

The role of recombinant insulin-like growth factor I in the treatment of the short child

PURPOSE OF REVIEW

Commercial preparations of recombinant human insulin-like growth factor I became available in 2005. Off-label use has been promoted because of the paucity of patients having approved indications; I review the background and rationale for such use.

RECENT FINDINGS

Attempts to identify growth hormone unresponsiveness in children with idiopathic short stature have been nonproductive. Treatment with insulin-like growth factor I for unequivocal growth hormone insensitivity improves but does not correct growth failure, in contrast to the typical experience with growth hormone replacement of growth hormone deficiency. This emphasizes the importance of direct effects of growth hormone at the growth plate, which cannot be duplicated by administration of recombinant insulin-like growth factor I. Adverse effects testify to the more than adequate delivery of administered recombinant human insulin-like growth factor I to other tissues, including lymphoid hyperplasia, coarsening of the facies, and increased body fat.

SUMMARY

In view of the risk profile, the limited ability of endocrine insulin-like growth factor I to restore normal growth, and the suppression of endogenous growth hormone (and therefore local effects on growth) that occurs with insulin-like growth factor I administration, the use of recombinant insulin-like growth factor I should be limited to those with well-documented growth hormone unresponsiveness and severe short stature.

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.

Pre-Analytic Considerations for the Proper Assessment of Hormones of the Hypothalamic-Pituitary Axis in Epidemiological Research

There is growing epidemiological interest in hormones as predictors of chronic diseases. The correct handling and analysis of hormones can be cumbersome, and great care must be taken in these processes in order to gain the most information possible. Given differences in sampling, processing, and stability of the various hormonal assays, we sought to provide a comprehensive review to aid future epidemiological research. We have coupled a thorough literature search with our own analytical experience to outline common laboratory problems one must consider in analyzing the hormones of the hypothalamic-pituitary axis. In addition, we describe the benefits and limitations of using alternative media--including urine, saliva, and blood spots on filter paper--to measure endocrine hormones in epidemiological studies.