Predictive values of erythrocyte indices for tests of iron, folic acid, and vitamin B12 deficiency.

The probabilities of low transferrin saturation, folic acid, or vitamin B12 levels in association with various erythrocyte indices was determined from the prevalences of these abnormalities and the distributions of the indices among 206 hospitalized and 1,000 ambulatory anemic patients. At mean corpuscular hemoglobin (MCH) greater than 30 pg, the probability of low transferrin saturation was 0.04 for hospitalized patients and 0.14 for ambulatory anemic patients. For MCH less than 27 pg, the corresponding probabilities were 0.52 and 0.67. The probabilities of low vitamin B12 or folic acid levels among hospitalized anemic patients were 0.0011 for mean corpuscular volume (MCV) less than 95 cu micron and 0.18 for MCV greater than or equal to 95 cu micron, indicating that measurements of these vitamins are of very limited value in most cases of anemia. These findings indicate that in some patients, the erythrocyte indices are sufficiently predictive for or against deficiency states to facilitate decisions regarding further diagnostic tests, as opposed to the increasing tendency to order such tests regardless of the indices.