PRELIMINARY EXPERIENCES WITH A BIOASSAY FOR ADRENOCORTICOTROPHIN (ACTH) IN UNEXTRACTED HUMAN PLASMA USING DISPERSED GUINEA-PIG ADRENAL CELLS

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Abstract
A simple, accurate and precise dispersed cell bioassay suitable for the assay of ACTH levels >50 ng/1 in unextracted human plasma is described. Isolated adrenal cells were prepared by tryptic digestion of the guinea‐pig adrenal gland and cortisol production by these cells was controlled specifically by ACTH. The cortisol response to 2 ng/1 of human pituitary ACTH (1–39) was always significantly (P < 0·05, n= 17) different from the control. ACTH concentrations between 1 and 20 ng/1 gave a 14 ± 3 fold (n= 17) increment in cortisol response. Dilutions of test plasmas were parallel with standard ACTH (1–39) when the plasma concentrations were maintained throughout at 4% (1 in 25 dilution) by the addition of dexamethasone‐suppressed plasma (‘ACTH free’). Using a 25 fold plasma dilution, the limit of detection of ACTH in unextracted plasma was 50 ng/1. Recovery of ACTH (1–39) spiked into dexamethasone suppressed plasma at concentrations of 250 and 100 ng/1 was 97·10% (n=17) and 114 ± 15% (n= 17) respectively. The within assay coefficient of variation (CV) of both quality control and patients' samples never exceeded 9% while between assay variation was 13%. The sample throughput was 30 plasma samples/week/technician.

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