Errors in plasma creatine kinase estimations on fetal blood samples resulting from contamination with amniotic fluid and maternal blood: relevance for the prenatal diagnosis of Duchenne muscular dystrophy.

This paper presents a detailed analysis of the calculation of fetal plasma CK activity in fetal blood samples contaminated with amniotic fluid and maternal blood. The seemingly simple formula for this calculation, first presented by Mahoney et al. (1977), is actually more complex than it appears; values for up to nine variables, two of which can only be assumed, are needed. Small variations in certain variables may result in very large errors in the final calculated value of fetal plasma CK activity. Examples of diluted blood samples are considered and the effects of allowing for reasonable errors in the variables is explored. The main source of error is in the measurement of CK activities in the diluted blood sample and in the amniotic fluid. Contamination by blood originating from the maternal circulation can also be a large source of error, especially if the mother is a carrier of DMD and maintains a high level of plasma CK activity during pregnancy. Fetal blood indices have to be assumed; these may be a source of significant error, depending on the difference between the actual and assumed values. The measurement of fetal plasma CK activity by the indirect calculation method is discussed in the context of the prenatal diagnosis of DMD.