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Hematocrit,% Reticulocytes, % Erythrocyte porphobilinogen deaminase and uroporphyrinogen decarboxylase were measured as previously described (5, 6) by using porphobilinogen and pentacarboxylporphyrinogen I, respectively, as substrates. CV5 for these assays in this laboratory are 8.3% and 14.2%, respectively. rocytes,it is possible that detection of these diseases could be compromised when coexisting conditions or interventions stimulate erythropoiesis. It will be of interest, for example, to determine if erythrocyte uroporphyrinogen decarboxylase increases during therapeutic phlebotomy in patients with porphyria cutanea tarda. The intent of this therapy, which is effective in both inherited and acquired forms of porphyria cutanea tarda, is to stimulate erythropoiesis and thereby utilize and reduce iron stores. If phlebotomy does increase erythrocyte uroporphyrinogen decarboxylase, it might be recommended that the enzyme activity be measured before institution of phlebotomy.

The peripheral blood film and the red blood cell size distribution histogram were examined for evidence of red blood cell fragments in 2,350 subjects. To distinguish subjects with greater than or equal to 10 fragments/1,000 red blood cells (abnormal) from normal, examination of the blood film was 0.83 sensitive and 0.30 specific, whereas examination of the histogram was 0.97 sensitive and 0.87 specific. The most common causes of abnormal fragmentation were malignancy with cytotoxic chemotherapy and severe iron deficiency. In two subjects, an abnormal red blood cell fragmentation pattern was the clue to a spectrin mutant in subjects with an automated blood count previously evaluated as normal. The data suggest two conclusions: the red blood cell volume histogram appears more accurate than the peripheral blood film for routine identification of red blood cell fragments; and asymptomatic spectrin abnormality identifiable by abnormal histogram may be a relatively common disorder.

In four patients with cold agglutinins, erythrocyte mean corpuscular volume was increased. Erythrocyte volume histograms showed that in each case macrocytosis was due entirely to doublet erythrocytes counted as single cells. For two of the four patients, macrocytosis was reported intermittently, when the blood had been allowed to cool to room temperature during laboratory processing. For these two patients, macrocytosis due to doublet erythrocytes could be elicited in vitro proportionate to the degree of cooling of the blood and could be abolished to rewarming to 37 C. Two other patients had macrocytosis constantly during the acute illness. Otherwise unexplained macrocytosis may reflect cold agglutinin activity without overt hemolysis, particularly if the macrocytosis is intermittent.