Examining cortisol ELISA in dried matrix spots: implications for analyte measurement and stability

Whole blood, plasma, whole milk and urine samples from domestic goats (Capra hircus) in both liquid and dried form were analysed by quantitative ELISA. Dried matrix samples were prepared using two types of absorbing support: glass fibre strip (strip-dried samples) and cellulose membrane (dried spots). Strip-dried samples have advantages over dried spots due to better cortisol recovery, reproducibility, ease of dried sample aliquoting and no need for recalculations against standard protocol. In the cortisol quantitative ELISA, the best concordance between the results was achieved in the pair "strip-dried whole blood/native plasma" and "strip-dried urine/native urine". The correlation coefficient of cortisol results in strip-dried blood, plasma and urine vs liquid samples ranged from 0.90 to 0.97 (p < 0.001). The mean recovery of cortisol from blood and plasma samples spotted on cellulose across a wide range of concentrations was much lower that from strip-dried blood and plasma, and varied between 40 and 60 %. Dried plasma samples with high cortisol content (>400 nmol/L) showed a slight decrease in the recovered cortisol concentrations on 20 % average. Cortisol in dried blood, plasma and milk samples was stable over a period of six months at 4 °C, a week at 37 °C or 24 h at 60 °C.