Should I use hemoglobin or hematocrit?

Although this will likely not result in life-or-death decisions and is more a matter of intellectual curiosity, I would argue that hemoglobin is superior to hematocrit because it is directly measured and tells you about O2 delivery. Read on.

Hemoglobin is directly measured with spectroscopy (it’s a pigmented dye, so can be directly measured).

Hematocrit can be measured, but in modern times is calculated (RBC count x MCV). It represents a ratio of packed cells: total volume. Conditions that change the osmolarity of the blood will change the MCV temporarily, and affect Hct. It is affected by multiple factors:

MCV:

  • Polycythemic or macrocytic anemias (larger MCV but less number of RBCs may show a normal Hct but low Hgb)
  • Microcytic anemias would show a reduced MCV and reduced RBC count, so the Hct should still be in line with the reduced Hgb

Hyperosmolar (falsely lowering the Hct):

  • DKA
  • Dehydration
  • Recent blood transfusion

Hypoosmolar (falsely elevating the Hct):

  • High altitude
  • Recent hemorrhage
  • Pregnancy

Some would argue that Hgb and Hct are the same. This paper in J Trauma showed that in their sample, Hgb and Hct had a Pearson’s R=0.99, which is pretty damn close to perfect correlation.

I’ve been told you shouldn’t order both Hgb and Hct, but you want to analyze RBC indices correctly, I think it’s necessary, as these depend on both Hgb and Hct.

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From Lab CE
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