Age has a pronounced effect on hematocrit, especially among children. The reference interval on the first day of life is 54 +/-10 (i.e., the reference interval is 44 to 64). By the fourteenth day, the interval is 42 +/- 7. By 6 months the reference interval is 35.5 +/- 5. The average hematocrit gradually increases through the teenage years, with the average reaching 39 between 11 and 15 years of age. Adult reference intervals are often stated as 47 +/- 5 for men and 42 +/- 5 for women.

Low barometric pressure has a pronounced effect on the reference interval for hematocrits. Native residents who live at high altitudes have generally higher hematocrits. The reference interval at about 4,000 ft, for instance, is 49.5 +/- 4.5 for adult men and 44.5 +/- 4.5 for adult women.

The reference interval for hematocrits is quite wide. Thus, if an individual begins near the upper end of the interval, as much as one fifth of the red cell volume may be lost before anemia can be demonstrated by a low hematocrit. Comparisons with previous hematocrits are important in assessing the development of anemia. For any one individual, the hematocrit is physiologically maintained within quite narrow limits. Thus, changes in hematocrit are a better diagnostic measure.

Hematocrit levels reflect the ratio of red blood cells to plasma. Thus diuresis will alter this ratio and thus the hematocrit by removing fluids without altering the red blood cell volume. In addition in acute blood loss whole blood is lost and the hematocrit will not reflect the blood loss until hours or days later when the plasma may expand to accommodate the blood loss.