Currently, the most reliable method for detecting cows with subclinical mastitis is to be enrolled in monthly testing with the Dairy Herd Information Association (DHIA).
For full article, https://hoards.com/article-37633-the-value-of-individual-cow-scc-data.html
By Jaimie Strickland, D.V.M., (Michigan State University College of Veterinary Medicine) for Hoard’s Dairyman