Evaluation of One Tube Osmotic Fragility as a Screening Test for Beta Thalassaemia Trait

Background: Thalassaemia is highly prevalent in Sri Lanka. The highest number of patients are seen in the North Western province, especially in the Kurunegala district. Screening of children and adults to detect beta thalassaemia carriers/trait using automated full blood counts and high performance liquid chromatography (HPLC) are only limited to few centers in Sri Lanka. In this context, the value of one tube osmotic fragility test (OFT) as a screening test is immense. Our study tried to determine the sensitivity and specificity of OFT in beta thalassaemia trait (BTT) in Sri Lanka.

Materials and Methods: This was a cross sectional study on randomly selected 700 subjects, carried out at National Thalassaemia Center (NTC), Kurunegala. Participants were categorized into four groups based on red cell indices, HPLC and serum ferritin assay as normal group, BTT group, iron deficiency anaemia (IDA) group and other haemoglobinopathies. OFT was performed on all cases.

Results: Out of 700 subjects 396 subjects (56.6%) were females and 304(43.4%) were males.

OFT gave definitely positive or equivocal results in 194 of 201 patients with BTT and 96.52% of sensitivity was observed. The test was false positive in 2 out of 268 (0.75%) normal subjects and 99.25% of specificity was observed. There were 3.48% (7/201) false negative results with a negative predictive value (NPV) of 97.44% and the positive predictive value (PPV) of the test was 98.97%.

Conclusion: One tube osmotic fragility test is a sensitive, cost effective, rapid and reliable primary screening test for detection of BTT in a population with financial restrictions.