The Hb in acute bleeding


Imagine you are called to review a patient who has started vomiting blood an hour ago. You arrive to find them tachycardic with a heart rate of 110. Amongst all the other stuff you would do for this patient, you check an Hb. Its 130, unchanged from baseline. What does this tell you about the severity of the bleed?

The answer is nothing at all. Zilch. De nada.

The reason for this is explained brilliantly in the chapter on acute blood loss anemia in the book Clinical Haematology: Theory and Procedures (1) and supported by more recent articles (2).

When blood is lost from vessels, it is both plasma and red cells that are lost in equal numbers. Therefore acutely the haemoglobin concentration will not change. What is responsible for the haemoglobin dropping is shift of fluid from the extravascular space to the intravascular space in response to reduced intravascular volume, thus diluting haemoglobin.  It can take 48 hours for the full effect of this to be seen.

The earliest haematological findings that are seen are actually an increase in the platelet count which can take place in as little as an hour. Soon after a neutrophilia with left shift of white cells develops. The latter can take 2-4 days to resolve.

So in the above example, don’t be reassured by the fact that the Hb is normal, even though many times even your seniors will try and tell you “the patient can’t be bleeding significantly because there is no haemoglobin drop”.  In fact, the presence of a resting tachycardia, as in the above example, indicates fairly significant blood loss, potentially consistent with greater than 750ml if the classification of shock systems are to be believed.

The other corollary to this is that you don’t base the decision to transfuse blood on the Hb level, but rather on the patient’s clinical progress/your assessment. The full ins and outs of this are something that is difficult to discuss in a blog post. Additionally, the presence of thrombocytosis or neutrophilia/left shift can give you clues to the presence of bleeding when you are unsure, as they develop quite early.


  1. Clinical Hematology: Theory and Procedures, Volume 936. Mary Louise Turgeon. Lippincott Williams & Wilkins, 1999. Pages 116-119.
  2. Clinical review: Hemorrhagic shock. Guillermo Gutierrez, David Reines and Marian E Wulf-Gutierrez. Critical Care2004(8):373