Cut-off values for diagnosis of G6PD deficiency by flow cytometry in Thai population

Abstract

In heterozygous females, X-inactivation causes a change in glucose-6-phosphate dehydrogenase (G6PD) activity from normal to deficient. Most G6PD screening tests are used to accurately diagnose hemizygous males, but they are less reliable for diagnosing heterozygous females. This study established flow cytometric cut-off values for screening of G6PD deficiency in hemizygous males and heterozygous or homozygous females. We studied 205 (125 females, 80 males) leftover blood samples from quantitative methemoglobin reduction (MR) screening. G6PD gene mutations determined by multiplex amplification refractory mutation system-polymerase chain reaction and direct DNA sequencing were used as the gold standard reference. Accuracy of the test, including the sensitivity, specificity, and positive and negative predictive values, was analyzed using MedCalc software. The optimal cut-off values for classification of %red blood cells with normal G6PD activity or %bright cells into homozygous normal, heterozygous, and homozygous deficiency in females were 85.4-100%, 6.3-85.3%, and 0-6.2%, respectively (sensitivity 93.2%, specificity 100%). The cut-offs for classification into hemizygous normal and hemizygous deficiency in males were 76.5-100% and 0-76.4%, respectively (sensitivity 100%, specificity 96.5%). Flow cytometry can be used to differentiate heterozygous females with intermediate phenotype from homozygous females, but cannot distinguish between heterozygous females with extreme phenotype and homozygous females. By flow cytometry, heterozygous and homozygous deficiency was detected in 29.6% and 3.2% of females, respectively. Among males, hemizygous deficiency was found in 31.3%. Flow cytometry can be used to screen patients with G6PD deficiency, and reliably and efficiently identify heterozygous and homozygous females, and hemizygous males based on cellular G6PD activity.

Keywords: Bright cells; G6PD mutation; Intracellular G6PD activity; Multiplex amplification refractory mutation system-polymerase chain reaction (multiplex ARMS-PCR); Quantitative MR; Thai.

Fig. 1

Dot plots show the distribution of %RBC with normal G6PD activity (%Bright cells) by genetic and quantitative MR test. Gray circles represent individuals who tested deficiency by quantitative MR. In females (A), dotted line at ≤6.2% bright cells represents the cut-off value of homozygous and dotted line at ≤85.3% bright cells represents the cut-off value of heterozygous as defined by multiplex ARMS-PCR/direct DNA sequencing. In males (B), dotted line at ≤76.4% bright cells represents the cut-off value of hemizygous as defined by multiplex ARMS-PCR/direct DNA sequencing

Fig. 2

Graphs show receiver operating characteristic (ROC) curve analysis for cut-off values of %bright cells between normal female versus heterozygous female (A), between heterozygous female versus homozygous female (B), and between normal male versus hemizygous male (C)

Fig. 3

Box plots show the mean (plus/minus standard deviation) %RBC with normal G6PD activity (%Bright cells) for each of the G6PD mutations in females (A) and in males (B)