Heterozygotes for the Mediterranean type of severe G-6-PD deficiency were investigated by a variety of tests.

The methemoglobin reduction test was most successful in detecting heterozygotes (about 80 per cent). Enumeration of methemoglobin containing cells on blood films (Kleihauer-Betke technic) did not improve these results. Quantitation of enzyme level was less successful (65 per cent), and determination of decolorization time by the BCB technic was least sensitive in heterozygote detection.

Methemoglobin reduction technics reflect a more indirect effect of the mutant gene than enzyme assay. The superiority of these technics in heterozygote detection is probably caused by the genetically determined presence of both normal and enzyme-deficient cells in G-6-PD deficient heterozygotes. Since methemoglobin reduction is carried out by individual cells, the population of enzyme deficient cells does not reduce methemoglobin, and therefore even a minority of deficient cells leads to abnormal test results. In contrast, enzyme assay is less successful for heterozygote detection, since measurement of enzyme level is carried out on hemolyzed red cells, where cellular mosaicism no longer exists.

An additional source of variation of enzyme levels in heterozygotes is caused by the existence of genetically determined control of normal enzyme level. Possession of a high capacity allele for G-6-PD activity may place a heterozygote in the normal range of enzyme activity.

The various tests were also applied to subjects with the mild Greek type of G-6-PD deficiency. Males with this mutation had enzyme levels varying between 12-45 per cent of the mean of normal males. Methemoglobin reduction test results were considerably less abnormal in hemizygotes with the mild type of Mediterranean deficiency than in heterozygotes with the severe deficiency. Fewer heterozygotes with the mild deficiency were detected.

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