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

Abstract

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 (t₁), 30 min after awakening (t₂) and in the evening (t₃) during 3 consecutive days. A fixed volume from each of the samples (t₁, t₂ and t₃) was pooled prior to laboratory analysis. Mean levels over 3 days for t₁, t₂ and t₃ 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.

• Cortisol
• HPA‐axis
• measurements
• methodology
• saliva
• stress

Acknowledgements

We thank all nurses and biomedical staff at the participating PHC centres. We also thank Iva Soderström and Agneta Berg in the Division of Clinical Chemistry at University Hospital, Linköping, who performed the laboratory analyses. Furthermore, we are grateful to Professor Holger Ursin for commenting on the work and the manuscript. The work was supported through grants from the Swedish Research Council (2004–1881) and the Swedish Heart and Lung Foundation (20040530). Grants from Linköping University and the County of Östergötland were provided via the two joint strategic programmes, CIRC (Cardiovascular Inflammation Research Centre) and the Department of Health and Society.