The utility of three different methods for measuring urinary 18-hydroxycortisol in the differential diagnosis of suspected primary hyperaldosteronism.

OBJECTIVE: Urine 18-hydroxycortisol (18-OHF) measurements are claimed to discriminate between primary hyperaldosteronism due to Conn's syndrome/adrenal adenoma or idiopathic bilateral adrenal hyperplasia (BAH), and also to identify cases of glucocorticoid-suppressible hyperaldosteronism (GSH). We have evaluated three urine 18-OHF methods using a panel of urine samples from patients with hypertension. DESIGN: Clinical methods comparative study. METHODS: Urine samples from patients with primary hyperaldosteronism due to either adenoma (n = 6), BAH (n = 6), GSH (n = 9), or essential hypertension (n = 38) were analysed without knowledge of the diagnosis using three different methods in different laboratories. These included 'in-house' radioimmunoassay (RIA), 'in-house' time-resolved fluorometric assay (DELFIA), and gas chromatography mass spectrometry (GC-MS). RESULTS: The three assays showed good correlation, but there were large bias differences: RIA bias was greater than DELFIA which was greater than GC-MS. Discrimination between adenoma and BAH patients was best for the DELFIA method, with no overlap between results for these two groups. All three methods gave significantly elevated results for the GSH group compared with the BAH and essential hypertension groups. No assay distinguished BAH from essential hypertension. CONCLUSION: Measurement of urine 18-OHF may be a useful additional test in the differential diagnosis of primary hyperaldosteronism. The clinical diagnostic value of urinary 18-OHF measurements is method-dependent with the DELFIA assay having the best discriminatory value.

Poor specificity and recovery of urinary free cortisol as determined by the Bayer ADVIA Centaur extraction method.

BACKGROUND: Immunoassay methods for urinary free cortisol (UFC) lack specificity, and many procedures have not been fully evaluated for routine use. In the current study we evaluated the Bayer ADVIA Centaur extraction UFC immunoassay and compared results to those obtained by a specific gas chromatography-mass spectrometry (GC-MS) method. RESULTS: The Bayer ADVIA Centaur cortisol assay lacked specificity. Cortisone, a steroid present in urine at concentrations similar to those of cortisol, demonstrated a cross-reactivity of 44%. In addition, the choice of matrix used to resuspend steroids after solvent extraction from urine affected recovery. Recovery was improved if the recommended urine reconstruction buffer was replaced by steroid-free serum. CONCLUSION: Irrespective of the sample matrix used, the Centaur method overestimates UFC, giving results up to twice those obtained by a specific GC-MS method.

Urinary transferrin, high molecular weight proteinuria and the progression of renal disease.

AIMS: Proteinuria predicts rate of progression in a variety of nephropathies. There is considerable evidence that iron-transferrin is toxic to proximal tubular cells in vitro, and recent clinical work suggests that selectivity of proteinuria influences the outcome of renal disease. The aim of this study was to examine the relationship between the nature of proteinuria and progression of renal disease. METHODS: This was a prospective, cross-sectional study in 66 patients with primary glomerulonephritis, diabetic nephropathy and a variety of other renal diseases. Urinary transferrin was measured by sandwich ELISA and correlated with rate of change in estimated creatinine clearance (ECC). Urinary SDS-PAGE was undertaken to divide proteinuria into tertiles according to molecular weight and to quantify the protein in each tertile. The magnitude of each tertile was then correlated with rate of change in ECC over a median period of 20 months. RESULTS: Rate of change of renal function correlated with total proteinuria (r2 = 18%, p < 0.001) and albuminuria (r2 = 17%, p < 0.001), but not urinary transferrin (r2 = 0%, p = 0.235). On univariate analysis high molecular weight proteinuria (r2 = 21%, p < 0.001), intermediate molecular weight proteinuria (r2 = 15%, p = 0.001) and low molecular weight proteinuria (r2 = 10%, p = 0.005) correlated with rate of change in ECC as did total fasting cholesterol (r2 = 7%, p = 0.003). On multivariate analysis, however, the only independent predictors of rate of change in ECC were high molecular weight proteinuria (r2 = 19%, p < 0.001), and total fasting cholesterol (r2 = 5%, p = 0.035). CONCLUSIONS: We found no evidence to support the hypothesis that iron-transferrin is important in the development of human renal injury. High molecular weight proteinuria correlates more strongly with rate of progression of renal disease than intermediate molecular weight, low molecular weight or even total proteinuria. This suggests either, that one or more high molecular weightproteins are implicated in causing progressive renal impairment, or that loss of size selectivity at the glomerular basement membrane is associated with accelerated tubulointerstitial damage.