Each udder quarter is separated from the others by membranes and normally no milk can pass from one udder quarter to another. The udder quarters are supposed to produce milk of equal amount and quality. In reality, there is a difference between front and rear quarters respectively. The rear quarters produces more milk of a lower fat content than the front quarters. The blood flow is shared by all four udder quarters so anything that affects the whole animal is likely to affect all four quarters equally. This means that a milk sample taken from all four quarters, composite milk, together with the information about the milk yield, is sufficient for calculating the energy requirements for the cow. If instead the sampling is made for udder health purposes, milk samples should be taken from each udder quarter separately. A quarter that suffers from inflammation produces less milk while the other healthy quarters produces as normal or perhaps compensates in milk production. The result is a dilution of the milk from the affected quarter. This means that mixing milk from all four quarters has a low diagnostic value, especially when the inflammation is mild or moderate. Due to the differences in milk production between front- and rear quarters, analyses for udder health purposes should include comparisons between front and rear udder quarters respectively to be accurate. http://www.miris.se/mastitis_indicator.htm