Method:
(1) Count the number of nucleated red blood cells in a given volume of blood within a large placental blood vessel.
(2) Convert the count to NRBC in 10^9/L.
(3) Estimate the NRBC count in the fetal blood based on the placental count. This is done using a linear regression equation comparing placental and cord blood counts.
The volume for counting is determined by a counting square, grid or circle. The simplest calculation is for the number of NRBC within an area delimited by a counting square.
volume in µL =
= ((side of counting square in mm)^2) * (thickness in microns) =
= ((side) ^2) * (thickness) * 0.001 cubic mm =
= ((side) ^ 2) * (thickness) * 0.001 µL
NRBC count per µL =
= (number NRBC counted in volume) / (volume in µL)
NRBC count in 10^9/L =
= (NRBC count per µL in the placenta) / 1000
LN(newborn NRBC count in 10^9/L) =
= (1.002 * LN(placental count in 10^9/L)) + 1.173 =
= X
newborn NRBC count in 10^9/L at birth =
= EXP(X)
Limitations:
• There are numerous sources of errors in the counts, and the correlation coefficient is mediocre (r-square 0.743). However, it can give a rough estimate of the count when no other data is available.
