Accurate light microscopic diagnosis of South-East Asian ovalocytosis

Introduction: Southeast Asian ovalocytosis (SAO) is a common inherited red blood cell polymorphism in Southeast Asian and Melanesian populations, coinciding with areas of malaria endemicity. Validation of light microscopy as a diagnostic alternative to molecular genotyping may allow for its cost-effective use either prospectively or retrospectively by analysis of archived blood smears. Methods: We assessed light microscopic diagnosis of SAO compared to standard PCR genotyping. Three trained microscopists, each assessed the same 971 Giemsa-stained thin blood films for which SAO genotypic confirmation was available by PCR. Generalized mixed modeling was used to estimate the sensitivity, specificity, positive predictive value, and negative predictive value of light microscopy versus “gold-standard” PCR. Results: Among red cell morphologic parameters evaluated, knizocytes, rather than ovalocytic morphology, proved the strongest predictor of SAO status (odds ratio [OR] =19.2; 95% confidence interval [95% CI] = 14.6-25.3; P = Conclusion: Light microscopic diagnosis of SAO by knizocyte visual cue performed comparable to time-consuming and costlier molecular methods, but requires specific training that includes successful differentiation of knizocytes from stomatocytes.