During the transition period, in early lactation, dairy cows experience the greatest economic losses, due to the appearance of diseases such as metabolic disorders, digestive disorders and reproductive disorders. One of the more common disorders in dairy herds with high milk production is ketosis, which is characterized by elevated levels of ketones in the blood, urine and milk. The period of early lactation is characterized by a negative energy balance (NEB), during which there is an increase in unsaturated fatty acids, and consequently the melting of fat reserves, which, due to the redirection of metabolism, turn into ketone bodies (beta-hydrobutyric acid (BHBA), acetoacetate and acetone), instead of serving for the formation of glucose. Ketosis can be primary or secondary, and depending on the clinical course: clinical or subclinical ketosis. Clinical signs of ketosis include decreased appetite, decreased milk production, weight loss, and hypoglycemia. Susceptibility to ketosis is probably a consequence of appetite restriction and increased energy needs of the mammary gland for vulnerable substances, especially glucose. The diagnosis of subclinical ketosis is more significant because ketone bodies appear in blood, milk and urine without the presence of clinical signs of the disease. The predominant ketone body in the bloodstream of ruminants and the most reliable and stable substance for diagnostics is beta-hydrobutyric acid (BHBA). The methods of determining BHBA are very simple and fast. Early detection of subclinical ketosis, with tests that determine an elevated level of BHBA, enables earlier intervention and reduces the loss of milk production. For the sake of prevention, this method of control should be introduced on every dairy cow farm, with an emphasis on checking cows up to 2 months after calving.