Although we often see the phrase somatic cell score (SCS), especially in articles estimating heritability of bull and cow, most readers do not understand how or why we use this form to view somatic cell count (SCC). To address this issue, we need to begin from the change of somatic cell numbers in milk. The somatic cell count of raw milk mainly consists of white blood cells and damaged or shed breast epithelial cells. For every milliliter (ml) of raw milk, the SCC for epithelial cells is usually below 5 million, accounting for about 2%, with the other 98% mainly being white blood cells. The most common reason for the increased SCC is when the breast is infected by bacteria. When the breast is infected, the immune system will send additional white blood cells to defend the bacteria, resulting in the increase of SCC for raw milk, which is why SCC is often seen as an important statistic for milk quality evaluation.

Of course this is not always the case, just like there are big variances from individuals of the same species.Since the SCC between individuals are usually up to several times in difference, we hope to convert the SCC number into a somatic cell score (SCS) between 0 to 9 in order to better view the data and group distribution. For instance, if the group average score is at 5, with a standard deviation of score 2, then there would be less than 20% of the group distributed below the score of 3. We achieve the transformation using a logarithmic conversion, turning the group data into a normal distribution, making it easier to read or further process the data.

Formula I.

Somatic Cell Score=log2（Somatic Cell Count/100,000）+3

Or in Abbreviation:

After numerous studies surrounding this topic, it has been most recognized to use logarithmic conversion with the base of 2 (formula 1.) In other words, for every increasing SCS, the respective SCC number will double (as shown in table 1.) From table 1, we can easily see that the SCS decreases as the SCC number decreases, and for every increasing SCS the amount of milk production decreases about 0.7 kg/day, or 180 kg/season. For bulls, the predicted transmitting ability for somatic cell score (PTA SPS) is preferred to be low, meaning that the expected SCC of its offspring to be lowered.

Welcome you to test : http://www.angrin.tlri.gov.tw/cow/cowscc_en.htm） 

Table 1. Conversion between SCC and SCS with rough estimation of correlated milk loss