Abstract
On four occasions over a period of four years samples of adult blood to which known amounts of fetal blood had been added were distributed to 8-12 different laboratories taking part in clinical trials organized by an M.R.C. Working Party. Estimates were made of the proportion of fetal: adult red cells in the samples after preparing films by the acid-elution method. When the proportion of fetal: adult red cells was less than about 1:10,000, the highest and lowest estimates were separated by a factor of about 10. However, when the number of cells present was between about 1:100 and 1:1,000, most results were between half and twice the true number of cells present.It is pointed out that since fetal red cells are approximately 30% larger than adult red cells, and since only about 90% of fetal cells stain darkly in the acid-elution method, estimates of the proportion of darkly-staining cells in a film underestimate the volume of fetal red cells present by about one-third. A simple formula is proposed which corrects for this factor and which gives an estimate of the total volume of fetal red cells present, deduced from the ratio of fetal: adult red cells and assuming a maternal red cell volume at term of 1,800 ml.A method of screening blood films is suggested which, firstly, endeavours to standardize the density of adult red cells on films, and, secondly, takes into account the Poisson distribution. Thus limits are set for the number of fetal red cells which can be seen in scanning a given number of adult cells before the suspicion is aroused that a transplacental haemorrhage exceeding a certain amount is present.It is emphasized that the density of adult red cells on blood films varies very widely, and unless the cell density and the size of the low-power field are defined the practice of deducing the extent of transplacental haemorrhage from the number of fetal red cells seen per low-power field may lead to large errors.