ACD: differential diagnosis especially from iron deficiency

The diagnosis of ACD is not easy and the differential diagnosis is wide including hemoglobinopathies, nutritional deficiencies, bleeding or hemolysis, medications, recurrent phlebotomy, and bone marrow infection or infiltration. Typically the anemia is mild to moderate, and the textbooks say normochromic and normocytic (although anemia may become microcytic as disease progresses). I think this is wrong and put about by academics who haven't spent thousands of hours looking at blood films of medical patients as I have. In my experience the anemia is often hypochromic and microcytic, reflecting the unavailability of perfectly adequate irons stores, though I conceed that concomitant iron deficiency may also be present (these patients are often treated with NSAIDS that cause gastrointestinal bleeding).

The reticulocyte count is low, reflecting the hypoproliferative nature of the anemia. Inflammation may be inferred from other features of the blood count, such as neutrophilia, monocytosis or thrombocytosis, and through measurement of non-specific inflammatory markers, such as C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR).

Exclusion of iron deficiency anemia is very important in the work-up of patients with ACD, although the two conditions frequently co-exist. Typically, serum iron and transferrin saturation are both decreased in ACD, indicating limited iron supply to the red cells, but transferrin levels are increased in IDA, whereas in ACD they are normal or decreased. Measurement of serum ferritin is frequently of little value, as ferritin is an acute phase protein as well as an indicator of iron stores, and levels will be increased in the presence of inflammation. The gold standard for assessment of iron stores remains a Perl’s stained bone marrow aspirate, but a bone marrow biopsy is otherwise of limited value in the diagnosis of ACD, so other non-invasive tools for measurement of iron supply are needed.

Serum transferrin receptor (sTFR) and sTFR/ferritin ratio

The measurement of sTFR, the truncated fragment of the membrane receptor, has been suggested as a possible tool for differentiating between ACD and IDA. The transferrin receptor is found on virtually all cells in the body, but is present at high levels on erythroid progenitors. sTFR levels increase in IDA as the availability of iron for erythropoiesis decreases, whereas in ACD levels may not differ from steady state because transferrin receptor expression is negatively affected by inflammatory cytokines. However, in practice interpretation of this assay in differentiating IDA from ACD has proved more difficult, and the assay has not been standardized. In CLL sTFR gives false values and is not used.

The ratio of sTFR to the log of the serum ferritin has been proposed to be a useful tool in the diagnosis of ACD, and particularly in differentiating ACD from IDA. A ratio <1 makes ACD likely, whereas ratios >2 suggest that iron stores are deficient, with or without ACD.

Red cell indices

Many modern haematology analysers are capable of calculating new red cell indices that may be useful in the evaluation of different forms of anaemia. Two of these, the reticulocyte haemoglobin content (CHr) and the percentage hypochromic red cells (%HYPO) (reported by Bayer Advia 120 haematology analyser) can provide information about iron supply to the erythron, and may be useful in guiding the management of ACD. CHr is a measure of haemoglobin in the most recently formed erythrocytes, while the %HYPO indicates the percentage of cells with haemoglobin content of <280 g/l. The former gives a relatively acute evaluation (48 h) of recent bone marrow activity, whereas the latter gives a time-averaged picture (20–120 d). Similar indices can be reported by the Sysmex XE-2100 analyser), which derives RET-Y (equivalent to CHr) and RBC-Y (equivalent to HYPO%). CHr has been shown to be a useful tool in the detection of early iron deficiency, as well as in monitoring early response to iron therapy.

A study has been made of the relationship between CHr, %HYPO and sTFR/ferritin ratio to evaluate anaemia in 442 patients with disease-specific anaemias and 154 non-anaemic subjects. A simple plot of CHr against sTFR/ferritin divided anaemic samples into four functional quadrants: (i) iron replete, normal eythropoiesis; (ii) reduced iron supply but not yet iron-deficient erythropoiesis; (iii) iron depleted with iron-deficient erythropoiesis; (iv) iron replete but with functional iron deficiency leading to decreased haemoglobinization. This may help in deciding whether iron supplementation may improve haemoglobin levels in individual patients. It is not known whether this will be valid in CLL patients.