The determination of 18-hydroxycorticosterone in saliva and plasma: comparison with aldosterone and glucocorticoids

A radioimmunoassay (RIA) for 18-hydroxycorticosterone (18OHB) is described using antibodies raised against 18OHB-3-CMO conjugated to bovine albumen and an iodine-125 labelled ligand. Extracts of plasma and saliva required thin-layer chromatographic purification prior to RIA. These assays have been validated in terms of precision, accuracy and specificity. The concentration of 18OHB in saliva is approx 20% of that in plasma and the two values are correlated. Thus, in a series of 20 healthy subjects the saliva and plasma concentration were 169 +/- 92 and 904 +/- 621 pmol.l-1 respectively with r = 0.72 (P less than 0.001). When the diurnal patterns of 18OHB, aldosterone and glucocorticoid (cortisol + cortisone) concentrations in saliva were investigated the patterns of 18OHB and aldosterone were usually similar though the 18OHB:aldosterone ratio can vary considerably. These and similar data suggest that the secretion of 18OHB and aldosterone are not completely synchronous. Our results show that saliva is a suitable body fluid in which to estimate 18OHB concentration and this measurement gives a useful reflection of the plasma 18OHB concentration. This technique should facilitate the investigation of the physiological role of this compound.