I have seen some references which say that ideally you should wait 24-48 hours (see Mollison - Blood Transfusion in Clinical Medicine), but that's not very real world. We try for 1/2 hour after the end of the transfusion if we receive an order that specifies a draw 'after the 2nd unit' (or 1st or whatever). However, if you are talking about a patient who is actively bleeding, the Hgb is a moving target, so an immediate (5 min) draw can be useful to the physician - is he keeping up with the bleed or losing ground. The amount of fluid pushed into your patient will also have an impact on the Hgb level - the Hgb level at 24 hours might be 10% higher than the level you got at 15 - 60 minutes due to fluid dynamics. So, in some cases, our draws are when the doc specifies. The time most commonly specified by our docs is 30 minutes. 5 minutes is common in surgical bleeds, 15 min for active GI bleeds. Many of our post-transfusion draws are simply ordered for next AM. You might find this article helpful: Equilibration of Hemoglobin Concentration After Transfusion In Medical Inpatients Not Actively Bleeding, Wiesen et al found in the Annals of Internal Medicine 15 august 1994, volume 121, Issue 4, Pages 278-280. Wiesen and company did a small study of their own with patients >18yrs and no recent active bleeding. They determined that Hgb levels measured in this type of patient at 15 minutes, 1 hr, 2 hrs, and 24 hrs were statistically equivalent. (They note small studies done in pediatric patients and neonates which give equivical results or agree with their findings in adults.) They also cite 19 sources, so you might find a study that is more relevant to your patient population.