A re-investigation of saliva collection procedures that highlights the risk of potential positive interference in cortisol immunoassay

Abstract

Background The use of saliva for measurement of cortisol permits non-invasive study of adrenal function, but collection can be technically difficult, particularly in small infants. Saliva collection can be assisted by citric acid to increase saliva flow, or by the use of cotton or polyester swabs in the mouth. Aim To determine whether different methods of saliva collection affect cortisol radioimmunoassay (RIA) performance. Experimental Cortisol was measured in saliva collected from 16 adults using intra-oral cotton swabs or polyester swabs, compared with saliva dribbled directly into a pot either alone (plain saliva) or after citric acid had been placed on the tongue. An in-house RIA, without prior extraction, was used to measure cortisol with an encapsulated sheep antibody. Results Mean (median) salivary cortisol was 10.9 (10.5) nmol L −1 in plain saliva, 10.4 (8.4) nmol L −1 in citric acid stimulated saliva; 25.3 (25.1) nmol L −1 in saliva collected on cotton swabs, and 27.9 (27.3) nmol L −1 collected on polyester swabs. Cortisol in saliva collected using citric acid was not significantly different from plain saliva ( p = 0.997), but cortisol in saliva collected using cotton and polyester swabs was significantly higher than that of plain saliva ( p < 0.01). Conclusion The use of cotton or polyester swabs for collection of saliva can result in spuriously high levels of cortisol when measured by RIA.