It quickly became apparent that this was unwieldy, and a different approach was developed that avoided the multiple reference range problem and also standardized results between laboratories, whose reference ranges varied a bit. In this approach, a median value for normal pregnancies is determined in each laboratory for each week of gestation in which AFP is measured. A patient's results are expressed as a multiple of this median, rather than as a raw AFP concentration. Thus, in a week with a median value of 42 ug/ml for maternal serum AFP, a value of 88 ug/ml would be expressed as an MoM of 2.1 (88 = 2.1 x 42) and would be elevated, as shown on the graph below. A value that is half the median (21 ug/ml) would be expressed as 0.5 MoM. Over a number of years of experience, it has been found that maternal weight, maternal race, and maternal diabetes mellitus (insulin-dependent) affect the AFP values associated with neural tube defects and Down's syndrome. Most laboratories today correct the MoM for these three conditions so that a given MoM value accurately reflects the risk of abnormality. Because of this correction, laboratories request that maternal weight, race, and diabetes status be provided, along with the gestational age, when the specimen is submitted.
It quickly became apparent that this was unwieldy, and a different approach was developed that avoided the multiple reference range problem and also standardized results between laboratories, whose reference ranges varied a bit.
In this approach, a median value for normal pregnancies is determined in each laboratory for each week of gestation in which AFP is measured. A patient's results are expressed as a multiple of this median, rather than as a raw AFP concentration. Thus, in a week with a median value of 42 ug/ml for maternal serum AFP, a value of 88 ug/ml would be expressed as an MoM of 2.1 (88 = 2.1 x 42) and would be elevated, as shown on the graph below. A value that is half the median (21 ug/ml) would be expressed as 0.5 MoM.
Over a number of years of experience, it has been found that maternal weight, maternal race, and maternal diabetes mellitus (insulin-dependent) affect the AFP values associated with neural tube defects and Down's syndrome. Most laboratories today correct the MoM for these three conditions so that a given MoM value accurately reflects the risk of abnormality. Because of this correction, laboratories request that maternal weight, race, and diabetes status be provided, along with the gestational age, when the specimen is submitted.