Is it possible to have at one lab: hematocrit 51.2%, Hemoglobin 17.8, at another lab 45% and 15.3 respectively (10 days difference). Both are reputable labs. Could hydration cause this discrepancy?
Red blood Cell (RBC) counts are affected by time of the day (diurnal variation), position, stasis, altitude, and hydration.
- Diurnal variation: Changes in hemoglobin (Hb) and RBC during the course of the day are usually slight, about 3%.
- Body position: Counts decrease slightly if you are in a recumbent position (leaning forward), and increase slightly when the patient is sitting upright. Changing from walking to lying down results in a 5–10% decrease in the Hb and hematocrit (Hct). Thus, you should rest for 5–10 min before blood collection. The difference in position of the arm during venous sampling, whether dependent or held at atrial level, can also affect the Hct.
- Leaving the tourniquet on for more than 60 seconds when obtaining a blood sample may increase the RBC count. Dehydration increases RBC count and over-hydration decreases it.
- Altitude: oxygen levels are lower at higher altitudes; thus, altitude reduces plasma volume, increases the Hb and Hct, and raises the number of circulating red cells. The magnitude of this variation depends on the degree of hypoxemia (low blood oxygen). At an altitude of 2000 m, Hb is 8–10 g/l and Hct is 0.025 higher than at sea level; at 3000 m, Hb is 20 g/l and Hct is 0.060 higher and at 4000 m Hb is 35 g/l and Hct is 0.110 higher. Corresponding increases occur at intermediate and at higher altitudes. These increases are due to both increased RBC synthesis (under the effect of low oxygen levels, so that the body can keep delivering adequate amounts of oxygen to tissues) secondary to the hypoxic stimulus and the decrease in plasma volume that occurs at high altitudes.
Red blood cells count is affected in the same day by dehydration and altitude posture.
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