A 70 kg adult with a bleeding peptic ulcer with 750 mL blood loss in the vomit ( 15% of blood volume) within the past 30 minutes will have postural fall in blood pressure due to acute volume loss, but will have normal values for hemoglobin.
Over the next 48 hours, most of the total blood volume deficit will be repaired by the movement of fluid from the extra vascular into the intravascular space. Only at these later times will the hemoglobin reflect blood loss.
However, if the total blood volume deficit is not fully repaired and the patient remains fluid deficient the hemoglobin will underestimate the degree of blood loss.
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