Q. Immediately after an acute episode of blood loss – following a motor vehicle accident, for example – the hemoglobin level is normal. Why is that? Are the other red cell indices normal too?

A. Immediately after acute blood loss, all the laboratory red cell indices are normal! The hemoglobin, RDW, RBC, MCH, and the MCHC are all normal. Because really, although the patient has lost blood (and therefore is in trouble because he/she has fewer red cells to transport oxygen through the body), the blood that’s examined in the laboratory appears totally normal! It has the same number of red cells per unit volume, and the red cells themselves are perfectly normal (assuming the patient’s blood was normal to begin with). This is because during acute blood loss, you’re losing red cells but also the plasma that goes along with them. So the blood remaining in the patient is totally normal – it’s just that there isn’t enough of it.

After a few hours (sooner, if you give the patient fluids), the blood will start to become more dilute as the patient pulls fluid from tissues into vessels. If you measure the hemoglobin (and the RBC) at this point, both will now appear decreased – and rightly so, because the total blood volume has now increased. The RDW, MCH, and MCHC, by the way, will be normal even at this point – because these tests measure the variation in size of the red cells (in the case of the RDW) and the amount of hemoglobin in each red cell (in the case of the MCH and MCHC). The patient’s problem is that there are not enough red cells around. The ones that are there, though, are completely normal.
Note: the elegant image of a single blood drop was taken by Abhishek Jacob and can be found at