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Real time analyses of main milk components are attended in milking parlours today. Breeders can know milk composition regularly. Energy (ketosis) milk quotients such as fat/crude protein (F/CP) and fat/lactose (F/L) could be essential for ketosis prevention and losses which can be linked with it. Aim was to do a reciprocal validation of reliability of milk indicators for subclinical ketosis (SK) identification. There were: dairy cows on 1st and other lactations free of clinical and superior subclinical mastitis; 3 herds of Czech Fleckvieh (CF) breed, 3 herds of Holstein (H) breed and 1 herd with CF and H; 329 individual milk samples from summer and winter season and 1st third of lactation. Average milk yield varied from 5,500 to 10,000 kg per lactation. Thresholds of F/CP and F/L were estimated (P < 0.05) according to SK milk acetone cut–off limit (≥10 mg.l−1). These were: for first lactation 1.27 (CF) and 1.32 (H), for other lactations 1.52 (CF) and 1.42 (H) in F/CP; for first lactation 0.84 (CF) and 0.84 (H), for other lactations 0.87 (CF) and 0.85 (H) in F/L. They were used for cross validation (P < 0.001) in the same order: 1.251, 1.31, 1.31 and 1.383 in F/CP; 0.831, 0.821, 0.989 and 0.852 in F/L. Validated cut–off limits are quite similar to their original values. It confirmes good reliability of original limits. The validated cut–off limits of milk indicators (F/CP and F/L) for SK in early lactation can be used at technological innovation in animal husbandry. Use of both quotients could improve the regular investigation of SK in practice.