Flow cytometric osmotic fragility testing does reflect the clinical severity of hereditary spherocytosis

Osmotic fragility (OF) testing based on flow cytometry (FCM) was recently introduced for the screening of hereditary spherocytosis (HS). This study was undertaken to compare the test efficiencies of the FCM OF test and the conventional OF test, and to investigate the correlation between FCM OF results and the clinical severity of HS.

The test efficiency of FCM OF test was retrospectively compared with one of conventional OF test. FCM OF test results are expressed in two ways, that is, as percentages of residual red cells (%RRC) in hypotonic saline (%RRC values) and by expressing as a healthy individual versus patient ratio of %RRC (%RRC ratio). Cutoff values were defined using 47 subjects including 22 HS patients. HS severity scores were determined using a scoring system devised to quantify the clinical severity of HS.

Using cutoff values of 1.68 for %RRC ratio and 61.9 for %RRC value, the test efficiency of %RRC ratio (93.6%) was higher than that of %RRC value (89.4%). However, their difference was not significant (P = 0.357). The FCM OF test (93.6%) achieved a higher test efficiency than the conventional OF test (68.9%) in 115 subjects, which included 75 HS patients (P = 0.002). %RRC ratios and %RRC values were significantly correlated with HS severity scores (P < 0.001 and P = 0.004, respectively).

The FCM OF test was found to have greater test efficiency than the conventional OF test. Furthermore, FCM OF test results quantitatively reflected the clinical severity of HS. Using %RRC ratio is recommended to minimize false positivity although its superiority over %RRC value could not be verified statistically. The FCM OF test could be the method of choice for routine diagnostic use to screen HS and assess its clinical severity.