The Laboratory and Clinical Investigation of an Absolute Erythrocytosis

The starting points in the further investigation of an absolute erythrocytosis are a knowledge of the causes of secondary erythrocytoses and the diagnostic criteria for PV (Tables 9-2 and 9-3, respectively). No single marker for the diagnosis of PV has, as yet, been identified. Currently patients must satisfy a number of diagnostic criteria and are said to have PV if they exceed a combination of "A" (major) and "B" (minor) criteria. Recent modifications to the PVSG, criteria have improved their sensitivity92 and are shown in Table 9-3. The modifications to the PVSG criteria add the demonstration of clonal hematopoiesis by detection of a karyotypic abnormality to the major criteria and replace leukocyte alkaline phosphatase (LAP) score and serum B12/unbound B12 binding capacity (UB12BC) with new minor criteria. These include radiological evidence of splenomegaly and the demonstration of either a reduced serum Epo level93 or evidence of endogenous erythroid colony growth in clonogenic assays in the absence of Epo stimulation.42 The reasons for these substitutions are that LAP scores show poor inter- and intralab-oratory reproducibility and that B12 assays are inaccurate at high levels, and few laboratories measure UB12BC.

The investigations may be undertaken in two stages (see Table 9-4). The first stage tests are mainly a battery of relatively simple tests to rule out secondary causes whereas the second stage tests are either more specialized, or are specific diagnostic criteria for PV.

Figure 9-3. Classification of the erythrocytoses.
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