Comparison of low ionic diluents for use with the Diamed antiglobulin gel test

Microtube column systems, although widely used in transfusion serology for the detection of red cell antibodies, may not detect weak Fy(a), Jk(a), S and K antibodies. A number of low ionic diluents are used to shorten the incubation time required for red cell antibody detection in the antiglobulin test. However, there are no published reports to show whether these low ionic diluents vary in their ability to detect red cell antibodies using microcolumn detection systems. Three low ionic diluents, Diamed ID-CellStab, Diamed ID-Diluent2 and an in-house produced low ionic strength solution (LISS), were assessed using the Diamed-ID LISS/Coombs microtube column system (in accordance with the manufacturer's instructions), to ascertain whether the choice of diluent influences red cell antibody detection. Two hundred and seventy patient samples were screened for red cell antibodies. The reaction strength was increased in 50% of the samples with detectable red cell antibodies using LISS as the diluent compared with ID-CellStab. Of the 51 red cell antibodies directed against Rhesus, Duffy, Kidd or Kell antigens, 21% reacted more strongly in LISS compared with Diamed ID-CellStab with a difference in grading of > or =1. Minimal disparity was found between ID-Diluent2 and LISS. Biochemical analysis of pH, osmolality, sodium, potassium and phosphate were comparable for ID-CellStab, ID-Diluent2 and LISS. Measurement of conductivity in each low ionic diluent was performed as a measure of ionic strength in the final reactant mix, as the same amount of low ionic diluent was used for each test. The conductivity was 3 x 5 mS cm for LISS and ID-Diluent2, and 5 x 8 mS cm for ID-CellStab; the acceptable range being 3 x 7 +/- 0 x 3 mS cm as cited in the Guidelines for the Blood Transfusion Services in the United Kingdom. This evaluation suggests that ID-CellStab is a suboptimal low ionic diluent for red cell antibody detection using Diamed-ID LISS/Coombs gel cards. The poorer performance of ID-CellStab compared with LISS may be explained by its higher ionic strength.