There are several ways that doctors commonly measure red blood counts:

• Red blood cell count. This test counts the number of red blood cells in a single drop (a microliter) of blood. As a general guideline, consider the normal range to be 4.5 to 6.2 million for men; 4.2 to 5.4 million for women.

• Hemoglobin. This test measures the red-pigmented protein that carries oxygen. Normal range for red blood count is 14 to 18 grams per deciliter in men; 12 to 16 grams per deciliter in women. When someone's hemoglobin drops below 10 grams per deciliter, he's considered anemic.

• Hematocrit. This test measures the percentage of total blood volume made up of red blood cells. Normal range is 40 to 52 percent for men; 35 to 47 percent for women.

A complete red blood count usually includes additional measures of the size, capacity, and number of red blood cells. The doctor will usually order a battery of tests to get a complete picture of the health of a patient's red blood cells. For example, a low hematocrit in combination with low hemoglobin may indicate iron deficiency, while a high hematocrit can indicate dehydration.