Is the turbidimetric immunoassay of haptoglobin phenotype-dependent?

Comparison of the turbidimetric immunoassay of haptoglobin with a reference method (the RID technique with appropriate correction for phenotype) clearly showed the turbidimetric assay to be phenotype-dependent. Correction factors for the three main phenotypes were calculated and reference values determined. The observed overall reference range of haptoglobin, irrespective of the phenotype, was 0.56-3.76 g/L. After correction for phenotype, reference values for the three types were: Hp 1-1: 0.77-2.49; Hp 2-1: 0.86-4.76; and Hp 2-2: 0.48-3.15 g Hp/L. From our reference values for the several phenotypes of haptoglobin it may be argued that phenotyping is desirable only when the uncorrected haptoglobin value, determined by turbidimetry, is within the range 0.38-0.98 g/L. Limited to hemolytic diseases, phenotype determination can be omitted beyond this range.