Hematocrit Facts

Hematocrit (Hkt, Hct, or Hk) refers to the volume fraction of cellular components in the total volume of blood.

Since erythrocytes (red blood cells) physiologically represent 99% of the total volume of blood cells, Hct is the percentage of the volume of all erythrocytes in the total blood.

The hematocrit describes the flow behavior of the blood or provides information about its degree of viscosity (viscosity):

  • The higher the hematocrit value, the higher the number of erythrocytes in relation to the blood plasma (liquid portion of the blood) → the blood is thus more viscous
  • The lower the hematocrit value, the lower the number of erythrocytes in relation to the blood plasma → the blood is thus less viscous

The procedure

Material needed

  • 3 ml EDTA blood (determination as part of the small blood count); mix tubes thoroughly by swirling immediately after collection.

Preparation of the patient

  • Not necessary

Disruptive factors

  • None known

Standard values

Men 40-54 %
Women 37-47 %

Indications

  • Basic diagnostics of hematopoiesis (blood formation).

Interpretation

Interpretation of increased values

  • Dehydration (lack of fluids).
  • Polyglobulia (high percentage of erythrocytes/red blood cells): think of polycythaemia vera (PV)!
  • Stays at high altitudes (e.g., high-altitude mountaineering) [values > 70%].
  • Blood doping (e.g., with epo/erythropoietin) [hematocrit limits in some sports federations (UCI and IAAF) set at: 50% for men and 47% for women; Nordic skiing by the FIS: 51.5% for men and 47% for women ]

Interpretation of lowered values

  • Erythrocytopenia (decrease in the number of red blood cells).
  • Hyperhydration – hypernatremia (excess sodium) with hypervolemia (total protein ↓); hematocrit ↓
    • Excessive saline intake:
      • Iatrogenic (caused by a physician).
      • Primary hyperaldosteronism (Conn syndrome)
    • Increased sodium reabsorption:
      • Impaired renal function

Further notes

  • Note: Even after a heavy bleeding, the hematocrit can be normal, because both cellular components and fluid are lost.