Predicting the reduction of parasitaemia following exchange transfusion in severe Plasmodium falciparum malaria: comparison of two mathematical formulae

Abstract

Two mathematical models to predict the level of parasitaemia after exchange transfusion in severe malaria have been described. One formula, described by Wilkinson and colleagues, calculates the level from the total volume of blood exchanged whereas the other, derived by Van den Ende and colleagues, is recursive and gives estimates of the reduction in parasitaemia after each aliquot of exchange. The accuracies of predictions based on these two formulae were compared using data collected from 20 patients undergoing partial exchange transfusion (40 ml blood/kg body weight). The transfusions led to significant changes in the mean (S.D.) haemoglobin concentrations, which rose from 8.9 (2.4) to 10.1 (1.5) g/dl, and in the median levels of parasitaemia, which fell from 16.5% (interquartile range = 12.8%–28.8%) to 4.5% (interquartile range = 1.2%–9.3%).

The median level of post-transfusion parasitaemia predicted by the Van den Ende formula (6.6%, with an interquartile range of 4.5%–10.2%) was similar to that observed, whereas that predicted by the Wilkinson formula (7.2%, with an interquartile range of 5.6%–12.4%) was significantly higher (P = 0.018). However, the median difference between the predictions based on the two formulae was represented by a parasitaemia of only 1.0% (interquartile range = 0.6%–1.85%). Thus, although the Van den Ende formula is more accurate than the Wilkinson, the difference is unlikely to be clinically significant.