Thioguanine nucleotides (6-TGN) and methylmercaptopurine (MeMP) are two important metabolites of thiopurine medications such as mercaptopurine (6MP) and azathioprine.
The laboratory tests for these metabolites are quoted in units of pmol/8×10^8 RBCs. (Picomoles per 800,000,000 red blood cells). Such as in this paper.
Why not quote the amount in units of 10^9 RBCs which would seem to be a more rounded number?