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Dry matter content and
its effects on fecal cortisol measurement: a critical assessment
E. Wilson, D.P. Dembiec,
K. Laughlin and A.J. Zanella
Michigan State University,
East Lansing, USA
Feces, is the biological sample of choice as a noninvasive measurement of glucocorticoids in some species. Various protocols abstain from using correction factors to relate fecal cortisol to circulating levels of hormone in the body. Protocols should take into account the total amount of fecal material excreted and the amount of dry matter present. We have found significant variation in dry matter to water ratio between individual animals, (60%-15% water/g feces). To test the implication of variability in dry matter, we spiked fecal samples from dogs and tigers to a final concentration of 0.012, 0.062; 0.123 and 0.610 ug/dl of cortisol above their endogenous levels. We compared the impact of adding different treatments of water (0%, 15%, 30%, and 60%) to freeze-dried spiked fecal samples to matched spiked control samples using our routine protocol, not freeze-drying. We reconstituted the samples using 2x1 (volume by weight) water and methanol (80:20). Samples were centrifuged and supernatant was transferred to RIA tubes (Diagnostic Product Corporation). Tubes were vacuum dried and reconstituted with water prior to running RIA. We found a significant difference in cortisol concentration between the control and the water added treatments in both dog (ANOVA; F4,25=24.29, p<0.001) and tiger samples (ANOVA; F 4,24 =7.84,p<0.001). After mathematically correcting for the dry matter content we found no significant difference between the control and the treatments in cortisol concentration in both dog (ANOVA; F 4,25 =3.27, NS) and tiger samples (ANOVA; F 4,24 =1.43; NS) indicating that our extraction protocol was unaffected by the water concentration in the samples. In conclusion, the process of drying the full sample is much more time consuming and cumbersome then calculating the dry matter content of a small aliquot of original sample and later making the mathematical correction for percent dry matter in the feces. Using the correction factor for dry matter may be helpful in relating fecal cortisol to circulating levels of the hormone.