Objectives Prevention of b-thalassaemia trait will, for the foreseeable future, hinge on effective screening strategies. Routine use of haematological data from automated cell counters may complement the results of the Naked Eye Single Tube Red cell Osmotic Fragility Test (NESTROFT), especially because of the high cost of a false-negative error. Our objective was to assess the potential additive value of routine haematological data in screening for b-thalassaemia trait. Settings Community survey of asymptomatic volunteers. Methods Using the NESTROFT results, haematological data and haemoglobin A 2 concentration from 1435 young, asymptomatic Sindhi subjects recruited in a population-based survey, and statistical analysis by classification tree approach, we examined whether haematological parameters have discriminatory utility additional to that of NESTROFT in screening for b-thalassaemia trait.Results We observed that in the derivation subset from which the classification tree was generated, there was only a marginal -albeit statistically significant -improvement in the screening performance of NESTROFT, whereas there was no such improvement attributable to the use of haematological parameters in a separate validation subset. Conclusion Our results further substantiate the claim that the use of NESTROFT is highly indicated for screening for b-thalassaemia trait in regions where the prevalence is high and the resources are constrained.

It is being increasingly recognized that a majority of the countries in the thalassemia-belt need a cost-effective screening program as the first step towards control of thalassemia. Although the naked eye single tube red cell osmotic fragility test (NESTROFT) has been considered to be a very effective screening tool for beta-thalassemia trait, assessment of its diagnostic performance has been affected with the reference test- and verification-bias. Here, we set out to provide estimates of sensitivity and specificity of NESTROFT corrected for these potential biases. We conducted a cross-sectional diagnostic test evaluation study using data from 1563 subjects from Central India with a high prevalence of beta-thalassemia. We used latent class modelling after ensuring its validity to account for the reference test bias and global sensitivity analysis to control the verification bias. We also compared the results of latent class modelling with those of five discriminant indexes. We observed that across a range of cut-offs for the mean corpuscular volume (MCV) and the hemoglobin A2 (HbA2) concentration the average sensitivity and specificity of NESTROFT obtained from latent class modelling was 99.8 and 83.7%, respectively. These estimates were comparable to those characterizing the diagnostic performance of HbA2, which is considered by many as the reference test to detect beta-thalassemia. After correction for the verification bias these estimates were 93.4 and 97.2%, respectively. Combined with the inexpensive and quick disposition of NESTROFT, these results strongly support its candidature as a screening tool-especially in the resource-poor and high-prevalence settings.

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