Evaluation of the single tube osmotic fragility test in detection of β-thalassaemia trait.

NESTROFT (the Naked Eye Single Tube Red Cell Osmotic Fragility Test) is a simpleand inexpensivemethod for detecting the β-thalassaemiatrait in the general population. Its results, however, depend on visual interpretation. We assessed interobserver variability in the recording of results by three individuals on 380 samples of blood collected intravenously. We also tested 745 samples by finger prick using a drop of blood or a uniform (20 μl) volume and found that when a uniform volume was tested the proportion of false positive results was7% (28/380)as compared to 22% (79/365) when a drop was used. We found that although there-was little variation in recording negative results, there was more variation in recording positive or doubtful results.We conclude that when the NESTROFT in field tests is positive or doubtful, then the blood should be further tested in a laboratory for Hb A2 and a uniform volume of blood rather than an arbitrary drop should be used in the test. NESTROFT is suitable for large scale preliminary screening for the β-thalassaemiatrait in India.