A crematocrit is an estimate of the fat and caloric content of a mother's milk sample obtained by spinning (using a centrifuge) a sample of mother's milk in a capillary tube much like blood is spun to obtain a hematocrit. It is most often used in a NICU setting to manage the breastmilk feedings of fragile, hospitalized infants.
Not all breastmilk has the same caloric content. It can vary from mother to mother, and at different times of the day from the same mother. It can vary from the start of a pumping or feeding session to the end and it can vary according to how long it has been since the last pumping or feeding. Lipid, or fat content is the primary component of mother's milk that changes causing the milk caloric content to vary. Most sources will say that breastmilk has 20 Kilocalories per ounce and it usually does if an entire 24 hours worth is combined and tested but it is possible for any given sample of milk to be somewhere in the 15 to 25 Kilocalories per ounce range, or to vary even more.
Why is this important? For a premature infant the typical weight gain goal is to maintain growth at about the rate of intrauterine growth. This is not always easy as the baby may have fluid restrictions as well as high metabolic needs. Sometimes these babies need more than 20 Calories per ounce to gain weight appropriately. It is possible using a crematocrit reading to calculate exactly how many Calories the mother's milk contains. If one also knows the typical times that milk is higher in fat (the end of pumping sessions and during pumping sessions with shorter intervals) then one can manipulate and fine tune which portions of the mother's milk is fed to the baby to provide the needed caloric content.
This is only possible when there is more mother's milk than the baby can consume. Using experienced guidance and some degree of individualized trial and error the mother will change the bottle she is pumping into partway through the pumping session. Milk fat increases in the later part of each session. The early or "foremilk" will be frozen to be used later and the higher fat "hindmilk" will be tested to determine the exact lipid percentage and therefore the exact Calorie content.
Luckily, most mothers of prematures can and should pump around 25 ounces a day by 2 weeks post partum if the pumping is managed correctly. This is much more than a premature baby will take until s/he is older but it is easier and best for the mother to establish the milk supply that will eventually be needed in the early weeks after birth than it is to try to increase the milk supply later.
There are other ways to increase the caloric content of breastmilk but they involve supplementing the milk with either Human Milk Fortifier (HMF)* or formula. Manipulating the milk aliquots given to the baby to only those containing higher milk fat percentages allows the normal ideal of exclusive breastmilk feeding. However, there are times when breastmilk must be supplemented with HMF to increase not only Calories but also protein and minerals. Even though the breastmilk of a mother who delivers prematurely is higher in protein and minerals in the first few weeks after delivery, very small premature infants require even more protein and minerals than they can get from their mom's breastmilk alone. Using hindmilk alone is not enough in these cases as protein and minerals do not vary in breastmilk the way lipids do. HMF adds either 2 or 4 Calories per ounce of breastmilk in addition to extra protein and minerals. If a small premature requires even more calories HMF cannot be used in larger amounts as it would overload the baby with certain minerals. In that case, hindmilk plus HMF might be the appropriate mix.
(the study that established correlation of lipids and caloric values of breastmilk:
Rush Mother's Milk Club info (Paula Meier contact info):
The machine used to spin and calculate: