Comparison of refractometer and biuret reaction as measurement methods for serum total protein concentration in Warmblood foals

This study compared two measurement methods to assess serum total protein (TP) concentration (refractometer and biuret reaction) in Warmblood foals on a farm with occurrence of equine proliferative enteropathy (EPE). The aim was to investigate the reliability of TP measurement by an automatic temperature compensated handheld refractometer in foals and its use as a diagnostic monitoring tool, for example in the context of a Lawsonia monitoring programme. In addition, the correlation between TP and albumin was investigated as well as the influence of the outdoor temperature on the measurement methods. Serum TP values by refractometry and by biuret reaction (n = 772) were compared in healthy and sick foals (n = 744), aged one to eight months. Additionally, the correlation between TP and albumin concentrations was analysed. The study results were assigned to one of two groups, group A or B. The samples from group A (outdoor temperatures = 29 degrees C) were taken over several weeks, whereas the samples of group B (outdoor temperature 33 degrees C) were all collected on one hot day. In both groups, the measurements were compared based on one single sample per foal. In group A (outdoor temperatures = 29 degrees C), serum TP median value by refractometer was 61 g/L (25 % and 75 % percentile: 58-64 g/L) and 59.8 g/L (25 % and 75 % percentile: 57-62.6 g/L) by biuret reaction. The TP values of both measurement methods, refractometry and biuret reaction, correlated positively with each other (r = 0.79). There was no significant difference (p = 0.98) between the two methods. Albumin values correlated positively with TP values in this group (refractometer: r = 0.19 and biuret reaction: r = 0.27). In group B (outdoor temperature 33 degrees C), serum TP median value by refractometry was 52 g/L (25 % and 75 % percentile: 50-55 g/L) and 59.9 g/L (25 % and 75 % percentile: 57.2-62 g/L) by biuret. The TP values of both methods correlated positively with each other (r = 0.87). In this group, TP values measured by refractometry were significantly lower than by biuret reaction (p <.0001). Albumin values correlated positively with TP values (refractometer: r = 0.14 and biuret reaction: r = 0.18). Comparing TP values with a pooled reference range for foals aged one to nine months (50-73 g/L), in group A (outdoor temperatures <= 29 degrees C), 98.4 % (1370/1392) of the values from both measurement methods were within the reference range, while 1.6 % (22/1392) were outside. 0.9 % (12/1392) of the values were in the range commonly observed in foals with Lawsonia intracellularis infection (< 50 g/L). In group B (outdoor temperature 33 degrees C), 87.5 % (133/152) of the values were within the reference range and 12.5 % (19/152) were outside. 12.5 % (19/152) of the values were in the range seen in foals with Lawsonia intracellularis infection (< 50 g/L). Group B (outdoor temperature of 33 degrees C) differed from group A (outdoor temperatures <= 29 degrees C) by noticeable deviating refractometrically measured TP values from chemically measured TP values. In general, the results of this study showed no significant difference between the two measurement methods. However, significant deviations occurred at outdoor temperature of 33 degrees C. Therefore, when using the refractometer, it seems to be important that samples are taken at moderate outside temperatures (<= 29 degrees C) or analysed promptly or stored refrigerated. Since only a small number of TP values was < 50 g/L, a statement of the comparability of the two applied measuring methods in the measuring range of interest for Lawsonia intracellularis is limited.