Laboratory Evaluation

Key laboratory tests in the office evaluation of the anemic patient include complete blood count (CBC), reticulocyte count, and iron studies (Box 39-1). The laboratory evaluation of anemia is designed primarily to determine effective response of the bone marrow to the anemia stimulus and to detect any disturbance in iron metabolism; this information allows physiologic classification of the most common anemias. The evaluation of the marrow's response to anemia is best approximated through corrected reticulocyte count (Fig. 39-2), which provides information about the number of newly released RBCs in circulation. Normally, a newly released RBC can be seen as a reticulocyte for about 24 hours. The reticulum of a reticulocyte is made up of residual ribo-somal ribonucleic acid (RNA); the cell is somewhat larger and appears bluer than mature RBCs on a Wright-Giemsa-stained peripheral blood smear.

First, however, the raw reticulocyte percentage must be corrected if it is to reflect the marrow production index. The first correction of the reticulocyte count is for dilution, as shown by the following equation for determining reticulocyte index (RI):

Box 39-1 Laboratory Tests in Office Evaluation of Anemia

1. Complete blood count (hemoglobin, hematocrit, red cell indices [MCV, MCH, MCHC], white cell count, and differential, platelet count)

2. Reticulocyte count (appropriately corrected)

3. Studies of iron status, including serum iron level, total iron-binding capacity (from which transferrin saturation [%] is calculated), and serum ferritin level

MCV, Mean corpuscular volume; MCH, mean cell hemoglobin; MCHC, mean cell hemoglobin concentration.

Figure 39-2 An increased number of reticulocytes are seen on a peripheral blood smear stained for reticulocytes. (From the American Society of Hematology image Bank image #1333. Copyright 1996 American Society of Hematology, used with permission.)

The normal reticulocyte count is 1% to 2% and the normal RI is 1.0. This correction is not necessary if the laboratory reports reticulocyte count as an absolute number, normally about 40,000 to 50,000 cells^L.

A further correction of the reticulocyte count is necessary if there is evidence from the peripheral blood smear that reticulocytes are being released prematurely from the bone marrow (shift cells or shift reticulocytes). Under these circumstances, reticulocytes live longer than the usual 24 hours in circulation, and thus the uncorrected reticulocyte count will overestimate the rate of new cell production. The second correction is shown by the equation for determining the marrow production index: MPI = RI/2. The normal MPI value is 1.0. The RI is divided by a factor of 2 to account for the prolonged reticulocyte life span in the circulation.

For example, if the reticulocyte count is 15% and Hct is 15%, there is evidence of shift reticulocytes on the peripheral smear. The MPI can be calculated as follows:

Also critical to understanding the pathophysiology of most anemias is to characterize the availability of iron for hemoglobin synthesis. This is done by measuring the serum iron level, total iron-binding capacity (TIBC), and serum ferritin level. Transferrin saturation percent is the proportion of iron binding sites occupied by iron atoms, reflecting the amount of iron immediately available for Hb synthesis, and is very labile. Serum ferritin is an indirect reflection of the body's total iron stores and is more stable. These values provide information about the two most common forms of anemia seen in the hospital or in the family physician's office—iron deficiency anemia and the anemia of chronic inflammation.

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