P42 A Simple Test for the Differential of Maternal and Fetal Bloods

Abstract

The structure of fetal haemoglobin (HbF) differs to that of adult haemoglobin (HbA) and protects it from denaturing when subjected to a pH change. With this knowledge, Apt et al1 adjusted the pH of the meconium, in melena neonatorium, by the addition of NaOH, and noted that a change of colour to green from pink occurred only if HbA was present. By utilising the same principle, the process can be modified to confirm the source of blood samples requested for the investigation of a possible fetomaternal haemorrhage, post delivery, for the prevention of Rhesus haemolytic disease of the newborn. 10 adult male bloods and six cord samples were used as the test material for the investigation. The rate at which the colour changed from pink to green of the test material was measured over a range of variables. The haematicrit was artificially adjusted between 30% and packed cells, with a permutation of volumes from 10 to 500 μL added to NaOH at concentrations between 0.05 M and 3 M.Results The optimum NaOH concentration was 0.1 M, equating to a pH 13.0 The haematicrit showed no influence on the reaction, but packed cells were chosen as the easiest to reproduce for consistency. The rate of colour change from pink to green took a maximum 1 min. Contamination of maternal with fetal bloods, and visa versa, at a low ratio is undetectable and at a high ratio gives a visual chimera of colour and intact erythrocytes.Method Using the above data a simple routine laboratory method of adding 100 μL of packed cells to 0.5 mL of 0.1 M NaOH, rapid mixing, and observing for a colour change between 1 min and a maximum of 10 mins. Pink to green show the blood to maternal i.e. HbA, but remaining pink indicative of HbF. The simultaneous testing of blood from and adult male and known cord blood, provides a positive and negative control respectively. This simple test has been extensively used to verify which are maternal and cord samples, when both express the same ABO and Rh(D) blood groups.1. Apt L, Downey WS Jr. (1953) Melena neonatorium: the swallowed‐blood syndrome. AMA Am J Dis Child.; 86(5): 639; discussion, 639–40.