Pooling ambulatory saliva cortisol samples over consecutive days - as reliable as arithmetic means.

OBJECTIVE When cortisol measurements are to be studied in large populations, cost-effective analyses are needed. This study aimed at testing whether one pooled cortisol value over three consecutive days is as reliable as using the arithmetic mean of the samples from the same measure points. MATERIAL AND METHODS Thirty participants aged between 45 and 69 collected saliva in salivettes immediately after awakening (t(1)), 30 min after awakening (t(2)) and in the evening (t(3)) during 3 consecutive days. A fixed volume from each of the samples (t(1), t(2 )and t(3)) was pooled prior to laboratory analysis. Mean levels over 3 days for t(1), t(2 )and t(3) were compared to corresponding levels of pooled vials. Cortisol levels were analysed using a radio immunoassay. RESULTS All measures tested had high correlations between mean values and pooled samples, exemplified with diurnal deviation r(dif t2-t3) = 0.974 (CI 0.946;0.987), and awakening response r(dif t2-t1) = 0.982 (CI 0.963;0.991). There were no statistical differences between the pooled values and the arithmetic means. CONCLUSION Pooling samples gave as reliable results as arithmetic means did. Pooling samples prior to laboratory analysis is a cost-effective method for measuring general diurnal cortisol variation in field research projects.