Validation and comparison of two methods of measuring lactate in equine plasma

Some methods of lactate (LA) measurement have not been validated appropriately for use in horses.

To validate 2 LA analysers (YSI 2300 Stat Plus and TDx Lactic Acid Assay) for use with equine plasma and to compare plasma [LA] determined by the 2 methods.

Both instruments were evaluated for linearity, parallelism, recovery and precision using serial dilutions of standard LA solutions and equine plasma and then comparing results with linear regression or paired t tests. Plasma [LA] results were compared in 275 blood samples collected from horses exercising at various intensities using Bland-Altman analysis. Level of significance was P < 0.05.

YSI exhibited good linearity for both LA standards and equine plasma (P < 0.05) at 0-30 mmol/l. TDx had good linearity at 0-12 mmol/l (P < 0.05); with LA standard solutions >12 mmol/l and with equine plasma, linearity was decreased. YSI exhibited good parallelism between LA standards and equine plasma LA measurements throughout the 0-30 mmol/l range (P > 0.05). Parallelism was poor with TDx (P < 0.05). Mean ± s.d. % recovery was 101.7 ± 3.4% for YSI (acceptable) and 110.6 ± 8.4% for TDx (unacceptable). Within-run and mean between-run coefficients of variation (CV) for plasma samples tested from 3.3-29.5 mmol/l were 0.4-3.0% for YSI. CVs for samples tested from 2.8-8.0 mmol/l were 17.4-24.1% for TDx. In 275 plasma samples, [LA] ranged from 0.1-42.7 mmol/l and 0.3-50.6 mmol/l for the YSI and TDx methods, respectively. The difference in plasma [LA] determined by the 2 methods was -1.0 ± 3.2 mmol/l, documenting that the TDx overestimated the YSI results by a mean value of 1 mmol/l.

It was concluded that the YSI method was a reliable method for measuring equine plasma [LA] from 0-30 mmol/l. The TDx method was found not to be suitable for use with equine plasma due to greater variability in measurements (high CV).