Atrial natriuretic peptide vs. N-terminal-pro-atrial natriuretic peptide for the detection of left atrial dilatation in horses

Reasons for performing study: Studies on the use of atrial natriuretic peptide (ANP) as a biomarker for left atrial dilatation in horses have produced variable results. Few have been performed, and the results may have been influenced by ANP instability, differences in sampling protocol and changes in the assay over time. N-Terminal proANP (NT-proANP) is a more stable molecule and might be a good alternative for clinical use. Objectives: To compare ANP and NT-proANP in terms of the detection of left atrial dilatation and to determine the influence of sample storage at temperatures of -80 and -20 degrees C. Study design: Prospective clinical study. Methods: Atrial natriuretic peptide and NT-proANP concentrations were compared between healthy horses (Group1, n = 20), horses with mitral valve regurgitation and a normal atrial size (Group2, n = 11) and horses with mitral valve regurgitation associated with left atrial dilatation (Group3, n = 16). The ANP concentration was measured with an equine enzyme-linked immunosorbent assay and the NT-proANP concentration with an enzyme-linked immunosorbent assay developed for use in human patients. Samples were stored at -20 and -80 degrees C and analysed within 7 months. Results: The NT-proANP concentrations were not significantly different between the groups. Horses in Group3 had a significantly higher ANP concentration (median 366pg/ml; interquartile range [IQR] 74-2000pg/ml) compared with horses in Group1 (median 31pg/ml; IQR 31-333pg/ml) or Group2 (median 31pg/ml; IQR 31-1152pg/ml; P = 0.02). The ANP cut-off value for detection of left atrial dilatation was 52pg/ml (sensitivity 81%; specificity 84%) for sample storage at -80 degrees C, and 44pg/ml (sensitivity 69%; specificity 84%) for storage at -20 degrees C. A larger decrease in ANP (45 126pg/ml) than in NT-proANP (10 +/- 31pg/ml) was found associated with sample storage at -20 instead of -80 degrees C. Conclusions: Atrial natriuretic peptide, but not NT-proANP, can be used to detect left atrial dilatation in horses. Atrial natriuretic peptide is less stable than NT-proANP when samples are stored at -20 degrees C. Atrial natriuretic peptide is a more suitable biomarker of left atrial dilatation in horses than NT-proANP.