A person who lies down experiences an increase in plasma volume as interstitial fluid enters the blood, which reverses after the person walks around. This can result in a drop in the hemoglobin and hematocrit, which may be misinterpreted as blood loss. The patient may undergo an evaluation for a non-existent anemia.



(1) The reference hematocrit and hemoglobin are measured after the person has been walking or standing for some time.

(2) The hemoglobin and hematocrit are then measured after the person has been recumbent for at least 20 minutes.


apparent percent drop in hematocrit =

= ((hematocrit when ambulant) - (hematocrit when supine)) / (hematocrit when ambulant)


This may range from 5-10% (Mollison) up to a maximum of 25% (Jacob et al).


Clinical findings:

(1) If the blood is resampled in an hour, no further drop in the hemoglobin and hematocrit has occurred.

(2) Retesting after the patient has had a chance to walk around shows a return to the reference level.

(3) The patient appears stable and the drop in the hemoglobin and hematocrit was unexpected. (The pseudoanemia may go undetected if the patient is unstable and/or blood loss is expected.)


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