On the validity of free hormone measurements

Interference in immunoassays to support therapeutic antibody development in preclinical and clinical studies

During preclinical and clinical studies, immunoassays are used to measure the concentration of the therapeutic antibody, anti-drug antibodies and soluble protein biomarkers. The reliability of these assays is crucial since the results are routinely used for safety assessment and dose selection. Furthermore, soluble protein biomarkers can provide information about target engagement, proof of mechanism, proof of principle and prediction of response. Study samples mostly consist of complex matrices that can exhibit considerable interference, resulting in inaccurate measurements. This perspective discusses the source of interference and strategies to mitigate or eliminate interference in immunoassays used during preclinical and clinical drug development of drugs with a focus on the development of therapeutic antibodies.

Real-time free cortisol quantification among critically ill children

OBJECTIVES

Ascertainment of adrenal function assessing free rather that total cortisol may be beneficial for the diagnosis of critical illness-related cortisol insufficiency. We hypothesized that centrifugal ultrafiltration would provide timely free cortisol data that highly correlated with the gold standard, but logistically cumbersome, equilibrium dialysis technique when the free cortisol fractions were identically quantified by chemiluminescence immunoassay. We also hypothesized that free cortisol would correlate with illness severity in a large cohort of critically ill children.

DESIGN

Prospective, multi-institutional, observational cohort investigation.

SETTING

Seven pediatric intensive care units within the Eunice Kennedy Shriver National Institute of Child Health and Human Development Collaborative Pediatric Critical Care Research Network.

PATIENTS

One hundred sixty-five critically ill children across the spectrum of illness severity.

INTERVENTIONS

Blood sampling.

MEASUREMENTS AND MAIN RESULTS

Time to derive plasma free cortisol concentrations after centrifugal ultrafiltration or equilibrium dialysis fractionation with chemiluminescence immunoassay was approximately 2 vs. approximately 24 hrs, respectively. Using centrifugal ultrafiltration, mean plasma free cortisol was 4.1 ± 6.7 μg/dL (median, 1.6 μg/dL; range, 0.2-43.6 μg/L), representing an average of 15.2 ± 9.4% of total cortisol. Nearly 60% of subjects exhibited free cortisol <2 and 30% <0.8 μg/dL, previously suggested threshold concentrations for defining critical illness-related cortisol insufficiency. Plasma-free cortisol concentrations comparing centrifugal ultrafiltration vs. equilibrium dialysis fractionation demonstrated a strong correlation (R2 = 0.97). For free cortisol <2 μg/dL, Bland-Altman analysis revealed minimal negative bias for the centrifugal ultrafiltration technique. Illness severity assessed by Pediatric Risk of Mortality III correlated moderately with free cortisol and percent total cortisol as free cortisol.

CONCLUSIONS

Determination of centrifugal ultrafiltration fractionated free cortisol was fast and results correlated highly with equilibrium dialysis fractionated free cortisol. Many children exhibited free cortisol <2 and <0.8 μg/dL but did not demonstrate clinical evidence of critical illness-related cortisol insufficiency. This study ascertains that real-time free cortisol quantification is feasible to potentially help guide clinical decision-making for cortisol replacement therapy in the pediatric intensive care unit.

Initial observations regarding free cortisol quantification logistics among critically ill children

PURPOSE

Corticosteroid insufficiency may occur among critically ill patients, but the diagnosis remains controversial. Historically assessment of free cortisol (FC) by means of equilibrium dialysis (ED) has required large blood volumes and prolonged fractionation time preceding analysis. We hypothesized that temperature-controlled centrifugal ultrafiltration with chemiluminescence immunoassay (CU/CI) would provide real-time FC data that highly correlated with ED/radioimmunoassay (ED/RI) or liquid chromatography/mass spectrometry (LC/MS) techniques.

METHODS

We quantified and correlated baseline and corticotropin-stimulated TC and FC by means of CU/CI, ED/RI, and LC/MS among healthy adults and 37 critically ill children.

RESULTS

Among critically ill children, FC was three- to fivefold higher than the healthy adults at baseline and increased another five- to eightfold following corticotropin administration. While TC increased approximately twofold following corticotropin administration, FC increased on average more than eightfold. Serum FC per CU/CI highly correlated with FC per ED/RI or LC/MS, but results were available in a fraction of the time. Children failing to increase TC by >9.0 μg/dL (248 nM) following corticotropin demonstrated an appropriate FC increase. Nearly 50% of critically ill children exhibited FC <2.0 μg/dL (55 nM). Neither FC nor TC concentrations correlated significantly with measures of illness severity.

CONCLUSIONS

Quantification of FC utilizing CU/CI was fast (1-2 h) and results correlated highly with ED/RI or LC/MS methodologies. These data require validation with larger cohorts of healthy and critically ill children but indicate that real-time FC quantification is available to guide cortisol replacement therapy.

Enhanced cortisol production rates, free cortisol, and 11beta-HSD-1 expression correlate with visceral fat and insulin resistance in men: effect of weight loss

Controversy exists as to whether endogenous cortisol production is associated with visceral obesity and insulin resistance in humans. We therefore quantified cortisol production and clearance rates, abdominal fat depots, insulin sensitivity, and adipocyte gene expression in a cohort of 24 men. To test whether the relationships found are a consequence rather than a cause of obesity, eight men from this larger group were studied before and after weight loss. Daily cortisol production rates (CPR), free cortisol levels (FC), and metabolic clearance rates (MCR) were measured by stable isotope methodology and 24-h sampling; intra-abdominal fat (IAF) and subcutaneous fat (SQF) by computed tomography; insulin sensitivity (S(I)) by frequently sampled intravenous glucose tolerance test; and adipocyte 11beta-hydroxysteroid dehydrogenase-1 (11beta-HSD-1) gene expression by quantitative RT-PCR from subcutaneous biopsies. Increased CPR and FC correlated with increased IAF, but not SQF, and with decreased S(I). Increased 11beta-HSD-1 gene expression correlated with both IAF and SQF and with decreased S(I). With weight loss, CPR, FC, and MCR did not change compared with baseline; however, with greater loss in body fat than lean mass during weight loss, both CPR and FC increased proportionally to final fat mass and IAF and 11beta-HSD-1 decreased compared with baseline. These data support a model in which increased hypothalamic-pituitary-adrenal activity in men promotes selective visceral fat accumulation and insulin resistance and may promote weight regain after diet-induced weight loss, whereas 11beta-HSD-1 gene expression in SQF is a consequence rather than cause of adiposity.

Measurements of serum free cortisol in critically ill patients

BACKGROUND

Because more than 90 percent of circulating cortisol in human serum is protein-bound, changes in the binding proteins can alter measured serum total cortisol concentrations without influencing free concentrations of this hormone. We investigated the effect of decreased amounts of cortisol-binding proteins on serum total and free cortisol concentrations during critical illness, when glucocorticoid secretion is maximally stimulated.

METHODS

Base-line serum total cortisol, cosyntropin-stimulated serum total cortisol, aldosterone, and free cortisol concentrations were measured in 66 critically ill patients and 33 healthy volunteers in groups that were similar with regard to sex and age. Of the 66 patients, 36 had hypoproteinemia (albumin concentration, 2.5 g per deciliter or less), and 30 had near-normal serum albumin concentrations (above 2.5 g per deciliter).

RESULTS

Base-line and cosyntropin-stimulated serum total cortisol concentrations were lower in the patients with hypoproteinemia than in those with near-normal serum albumin concentrations (P<0.001). However, the mean (+/-SD) base-line serum free cortisol concentrations were similar in the two groups of patients (5.1+/-4.1 and 5.2+/-3.5 microg per deciliter [140.7+/-113.1 and 143.5+/-96.6 nmol per liter]) and were several times higher than the values in controls (0.6+/-0.3 microg per deciliter [16.6+/-8.3 nmol per liter], P<0.001 for both comparisons). Cosyntropin-stimulated serum total cortisol concentrations were subnormal (18.5 microg per deciliter [510.4 nmol per liter] or less) in 14 of the patients, all of whom had hypoproteinemia. In all 66 patients, including these 14 who had hypoproteinemia, the base-line and cosyntropin-stimulated serum free cortisol concentrations were high-normal or elevated.

CONCLUSIONS

During critical illness, glucocorticoid secretion markedly increases, but the increase is not discernible when only the serum total cortisol concentration is measured. In this study, nearly 40 percent of critically ill patients with hypoproteinemia had subnormal serum total cortisol concentrations, even though their adrenal function was normal. Measuring serum free cortisol concentrations in critically ill patients with hypoproteinemia may help prevent the unnecessary use of glucocorticoid therapy.