Relationship between the Sodium Nitroprusside Test for Ketone Bodies in Urine and Serum β-Hydroxybutyrate Concentrations in Dairy Cows

The relationship between the sodium nitroprusside (SNP) test for ketone bodies in urine and concentrations of serum ß-hydroxybutyrate (ß-HB) was studied in 50 Holstein Friesian cows on a commercial dairy farm, including 10 cows in the dry period, and 22, 8 and 10 cows and in week 2, 4 and 8 of lactation, respectively. Urine and blood samples were collected from all cows and were transported to the laboratory on the sampling date. Urine samples were analyzed for ketone bodies using the SNP test within 24 hours after collection. Serum samples were analyzed for the concentration of ß-HB. Results of the SNP test for ketone bodies were classified as either N (negative), +1 or + 2 based on the color reaction of SNP with a known amount of acetone in aqueous solution at 0 mmol (N), 1 to 5 mmol (+1) and 6 to 10 mmol (+2) respectively. Results of the SNP test for ketone bodies in urine were highly correlated with the concentrations of ß-HB in the serum (r = 0.82). Concentrations of serum ß-HB were 0.46 ± 0.03, 0.98 ± 0.10 and 2.63 ± 0.34 mmol/L for cows with the SNP test results of N (n = 31), +1 (n = 10) and +2 (n = 9), respectively. Concentrations of serum ß-HB were 0.42 ± 0.04, 0.97 ± 0.21, 1.48 ± 0.44 and 0.89 ± 0.12 mmol/L for cows in the dry period and in week 2, 4 and 8 of lactation, respectively. The method was practical, simple and inexpensive and therefore, it would be very beneficial for the diagnosis of subclinical ketosis or the early detection of ketosis in dairy cows, particularly on smallholder farms.