Affecting hemoglobin test for results can be caused by a number of physiological factors and sampling errors. It is important to establish a standardized procedure when measuring hemoglobin.
Physiological factors on Hemoglobin test for results
| Gender | For a given finger stick result, the expected venous Hb value is 0.5 to 0.8 g/dL lower for women compared to men. |
| Source of sample |
Capillary blood has higher Hb than venous blood, especially in women and in men with severe iron depletion (median +0.67 g/dL or +6.7 g/L for iron-depleted women to -0.1 g/dL or -1 g/L for iron-depleted men). Venous blood has a slightly higher Hb than arterial blood. |
| Sampling site | Ear stick sampling has been used in the past but has been shown to produce values that are higher than venous and finger stick values. Finger stick sampling has been shown to more closely approximate venous Hb values. |
| Tourniquet use | Tourniquet use longer than 30 seconds increases venous hemoglobin value. |
| Body position | Hb is higher in blood samples from standing subjects than in samples from sitting or supine subjects. For example an increase of up to 9% after 15 minutes of standing or a decrease of 2.4-2.7% when moving from a standing to a seated position can be seen in venous hemoglobin values. |
| Diurnal variation | Hb tends to be higher in the morning and decreases throughout the day. |
| Dehydration | Loss of plasma volume for example, due to transpiration or insufficient fluid uptake causes increased hematocrit and hemoglobin values. |
| Altitude | The normal hemoglobin concentration increases at high altitudes (>1,500 m) to compensate for the lower concentration of oxygen in the air. This effect becomes more pronounced with increasing altitudes and should be corrected for when interpreting hemoglobin results . |
When carrying out comparative Hb testing for studies or evaluations, the samples should be taken under identical conditions.
Capillary sampling technique
Both accuracy and reliability of hemoglobin measurements can be affected by pre-analytical errors. Following a standardized procedure, as well as operator training and practice, is essential to obtain correct results especially from capillary sampling (finger sticks).
Potential sources of error are:
| Choice of lancet | The lancet must make a sufficiently deep puncture to ensure an adequate flow of blood (1.85 to 2.25mm is recommended, depending on the thickness of the skin). |
| Selection of puncture site | The middle or ring finger should be used, ideally of the non-dominant hand, as they are generally less calloused and less sensitive to pain compared to the index finger or thumb. The thumb should also be avoided due to its pulse (arterial presence). In the fifth finger the distance between the skin surface and the bone is too small. The puncture should be made slightly off center from the central, fleshy portion of the fingertip – near the side, but not on the very side of the finger. The hand must be warm and relaxed. The patient must not wear a ring on the finger as this may obstruct the blood circulation. |
| Cleaning & disinfection | After cleaning and disinfection, the puncture site must be dried completely. Remnants of alcohol solution will dilute the blood and cause false low readings. |
| Puncture | The finger should be supported by the operator´s hand. It can be massaged gently before and after the puncture to stimulate blood circulation. Maintaining a light pressure in the moment of the puncture helps to achieve a good penetration. |
| Capillary flow | The first 2-3 drops of blood should be wiped away using a cle an gauze pad. Thinning and clotting of the blood must be avoided as it causes incorrect results. A good capillary flow is normally found within 30-45 seconds after the puncture. The 3rd or 4th drop of blood should be used to fill the cuvette for the hemoglobin measurement. The drop must be big enough to fill the cuvette completely. Incomplete filling or air bubbles cause false results. The finger must not be squeezed hard or “milked” to increase the size of the drop as this will dilute the sample with interstitial fluid. |
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