The relationship between hematocrit and viscosity is well-known, however, the relationships between white blood cell (WBC) or platelet count and viscosity were not fully studied. 18 A nomogram for this purpose is provided in Figure 1.Blood viscosity is correlated with cerebral blood flow and cardiac output, and increased viscosity may increase the risk of thrombosis or thromboembolic events. The probability of iron deficiency can usually be established by correcting an individual ferritin value for the degree of coexistent inflammation as indicated by the ESR, possibly avoiding a bone marrow examination. 17 In the past, the final arbitrator in this situation has been bone marrow aspiration with Prussian blue staining to determine the presence of iron. Similarly, an individual serum ferritin level may not be helpful when inflammation is present because ferritin is an acute phase reactant and may be artifactually elevated. Because both may have a transferrin saturation of around 15 percent, simply evaluating the serum iron level and percent saturation will not differentiate between the two conditions. Unfortunately, neither iron studies nor serum ferritin levels are definitive in distinguishing between these two types of anemia. 17, 18 Iron deficiency anemia and anemia of chronic disease are hyporegenerative and characterized by a low reticulocyte count. The ESR may be useful in differentiating iron deficiency from anemia of chronic disease in patients with a background chronic inflammatory condition such as rheumatoid arthritis. When an increased rate is encountered with no obvious clinical explanation, the physician should repeat the test after an appropriate interval rather than pursue an exhaustive search for occult disease. An extreme elevation of the ESR is strongly associated with serious underlying disease, most often infection, collagen vascular disease or metastatic malignancy. Some studies suggest that the test may be useful as a “sickness index” in the elderly or as a screening tool for a few specific infections in certain settings. Use of the ESR as a screening test to identify patients who have serious disease is not supported by the literature. It is useful in monitoring these conditions and may predict relapse in patients with Hodgkin's disease. The test remains helpful in the specific diagnosis of a few conditions, including temporal arteritis, polymyalgia rheumatica and, possibly, rheumatoid arthritis.

However, the usefulness of this test has decreased as new methods of evaluating disease have been developed. The erythrocyte sedimentation rate (ESR) determination is a commonly performed laboratory test with a time-honored role.

Test viscosity of hemoglobin in rbc