Subclinical ketosis is defined as high serum ketone body concentrations without observed clinical signs. Subclinically affected cows are at increased risk of clinical ketosis and displaced abomasum and are also less fertile than those with normal serum ketone body concentrations. Furthermore, they appear to have reduced milk production. Determination of serum β-hydroxybutyrate (BHB) concentrations is considered the best way to detect and monitor subclinical ketosis, because the cow-side tests mentioned above are insufficiently sensitive and specific in detecting subclinical increases in serum BHB concentrations. Serum concentrations may be determined spectrophotometrically by traditional clinical laboratory means. The BHB concentrations in blood or serum samples are reasonably stable; thus, rigorous sample handling precautions are not necessary to transport the specimens to the laboratory. The test is sensitive to hemolysis, however, so hemolysis should be avoided during sample collection, and serum should be separated from the clot before shipment to the laboratory.
In addition to laboratory determination by spectrophotometry, handheld devices manufactured to monitor blood ketone body concentrations in human diabetic patients have been evaluated for use in monitoring subclinical ketosis in cows. These instruments use whole blood rather than serum for BHB determination, making them particularly practical for on-farm use. The whole blood BHB concentration is very close to the serum concentration, so the interpretation of results obtained from either the handheld device or laboratory analysis is similar.
Diagnosis of subclinical ketosis requires definition of a concentration above which cows are considered to be subclinically ketotic. Concentrations between 1,000 µM (10.4 mg/dL) and 1,400 µM (14.6 mg/dL) are used. Recommended strategies for herd-level testing are to test at least 12 animals in the first 60 days of lactation. If >10% are subclinically ketotic, it should be considered evidence of a herd-level problem and prompt a review of nutritional management. Some farms use the handheld BHB meters to test all cows in early lactation. Cows diagnosed with subclinical ketosis are treated with propylene glycol. Such an approach is labor intensive but has been demonstrated to reduce further disease occurrence in subclinically ketotic animals and to improve milk production in treated animals. Such an approach should not replace sound nutritional management procedures.