(Medical audience) Full Blood Counts

Dee GP

For patients:

Full blood count (FBC) samples are collected into tubes containing an additive called EDTA, which stops the blood from clotting in order to count the different types of cell. While a sample is in transit, red blood cells gradually take up a small amount of fluid, causing them to swell slightly.

This effect is more pronounced for us than for most practices, because our samples travel by post rather than by the same-day courier most laboratories use for local collections — so the transit time, and the resulting effect, is somewhat longer. This can produce a small, expected rise in the measured red cell size (MCV) and related calculated values, without reflecting any real change in your blood.

“Full blood count (FBC) samples are collected into tubes containing a preservative called EDTA, which stops the blood from clotting so the different cell types can be counted. While a sample is in transit — for example, when posted to the laboratory — the red blood cells gradually take up a small amount of fluid, causing them to swell slightly. This can produce a small, expected rise in the measured red cell size (MCV) and related calculated values, without reflecting any real change in your blood. Laboratories are familiar with this effect and take it into account when reviewing results from posted samples.”

The analyser directly measures Hb, MCV and RBC. Everything else is calculated.

In base SI units these all balance exactly. With conventional units (Hb in g/L, MCV in fL, RBC in ×10¹²/L), the Hct calculation comes out 1000x too large — divide by 1000 to get Hct as a fraction.