Comparing equations to correct total cortisol and total thyroxine hormone concentrations using haematocrit and total serum proteins in haemodiluted anesthetized cats

Objective To determine if haemodilution affects total cortisol and total thyroxine concentrations and, if so, to determine which equation and variables used in the correction factor would be useful to detect a change in concentration potentially masked by dilution. Study design Retrospective study on banked frozen samples. Animals Six cats, 21 ± 1 months old (mean ± standard deviation) and 4.9 ± 1.2 kg. Methods Total cortisol, total thyroxine, total serum protein (TSP) and haematocrit (HCT) were measured pre-haemorrhage and 30 minutes after haemodilution [haemorrhage: 23.5 (19.6, 27.5) mL kg -1 ; Fluid: 22.5 (15.9, 29.1) mL kg -1 ]. Equations used to correct diluted hormone concentrations were: (A) MSHC × [(pre-haemorrhage X)/(haemodiluted X)] (B) MSHC × [(1 - haemodiluted X)/(1- pre-haemorrhage X)] (C) MSHC × {[(1 - haemodiluted X) × pre-haemorrhage X]/[(1 - pre-haemorrhage X) × haemodiluted X]} Where MSHC is measured serum hormone concentration, X is either HCT (L L -1 ) or TSP (g L -1 ) used in the correction factor. Corrected hormone concentrations were compared to pre-haemorrhage concentrations using a repeated measures ANOVA, agreement using repeated measures Bland-Altman, and correlation using concordance correlation coefficient. Significance was p < 0.05 and data reported as mean (95% confidence interval). Results Pre-haemorrhage cortisol was 69 (47, 92) nmol L -1 and not significantly different to the haemodiluted value of 63 (44, 82) nmol L -1 ( p = 0.313). Pre-haemorrhage thyroxine was 37 (31, 42) nmol L -1 and significantly higher than 19 (16, 22) nmol L -1 for the haemodiluted value ( p < 0.001). Equation A using HCT identified a masked increase in cortisol [104 (78, 129) nmol L -1 ] and similar thyroxine [33 (27, 38) nmol L -1 ] concentrations. Conclusion and clinical relevance Hormone concentrations are affected by haemodilution and require correction. Equation A using HCT corrected hormone concentrations that approximated expected changes in concentration in response to haemorrhage and haemodilution.