what is the concentration of methemoglobin inblood (gm/dl)to say that's cyanosis?

Question

i know that if the deoxygenated blood if below 85%is cyanosis ,but how much it is in gm/dl?

Answer 1

Cyanosis becomes apparent when the mean capillary concentration of reduced hemoglobin exceeds 5 grams per deciliter. It is the absolute rather than the relative amount of reduced hemoglobin which is important in producing cyanosis.
Clinical note - since you sound like a medical student - Remember that you will not see cyanosis when the percentage of oxygenated blood is below 85% "IF" there is also anemia.
Conversely, if the hgb/hct are elevated in erythrocytosis or polycythemia, you may see cyanosis when the oxygen saturation is above 85%.
Cyanosis is a clinical finding - an observation - it is not a defined laboratory parameter. 'Tis in the eye of the beholder.