Tag Archives: hemoglobin

Serial Hemoglobin / Hematocrit – Huh? Part 1

The serial hemoglobin (Hgb) determination. We’ve all done them. Not only trauma professionals, but other in-hospital clinical services as well. But my considered opinion is that they are not of much use. They inflict pain. They wake patients up at inconvenient hours. And they are difficult to interpret. So why do them?

First, what’s the purpose? Are you looking for trends, or for absolute values? In trauma, the most common reason to order is “to monitor for bleeding from that spleen laceration” or some other organ or fracture complex. But is there some absolute number that should trigger an alarm? If so, what is it? The short answer is, there is no such number. Patients start out at a wide range of baseline values, so it’s impossible to know how much blood they’ve lost using an absolute value. And we don’t use a hemoglobin or hematocrit as a failure criterion for solid organ injury anymore, anyway.

What about trends, then? First, you have to understand the usual equilibration curve of Hgb/Hct after acute blood loss. It’s a hyperbolic curve that reaches equilibrium after about 3 days. So even if your patient bled significantly and stopped immediately, their Hgb will drop for the next 72 hours anyway. If you really want to confuse yourself, give a few liters of crystalloid on top of it all. The equilibration curve will become completely uninterpretable!

And how often should these labs be drawn? Every 6 hours (common)? Every 4 hours (still common)? Every 2 hours (extreme)? Draw them frequently enough, and you can guarantee eventual anemia.

Bottom line: Serial hemoglobin/hematocrit determinations are nearly worthless. They cost a lot of money, they disrupt needed rest, and no one really knows what they mean. For that reason, my center does not even make them a part of our solid organ injury protocol. If bleeding is ongoing and significant, we will finding it by looking at vital signs and good old physical exam first. But if you must, be sure to explicitly state what you will do differently at a certain value or trend line. If you can’t do this and stick to it, then you shouldn’t be ordering these tests in the first place!

In my next post, I’ll discuss a newly published paper that objectively shows the (lack of) utility of this testing method.

Cool Device: Noninvasive Spot Check Hemoglobin

I always tell my trainees that “your patient is bleeding to death until you can prove otherwise.” Sometimes bleeding is obvious in our trauma patients and sometimes it isn’t. The usual routine for assessing major trauma patients involves a blood draw, with a high priority on obtaining a specimen for the blood bank. But most centers also get standard analyses on the blood, including CBC, lytes, etc.

But remember, a blood draw is a snapshot. And it’s a snapshot of values that change relatively slowly. This means that you can get suckered into believing that your patient is okay because one set of labs looked pretty normal. And it’s impractical (and uncomfortable) to get labs frequently with repeated needle sticks.

Masimo, a medical equipment manufacturer, has added something extra to the pulse oximeter that you are already familiar with. Using the usual clip-on finger probe, it measures arterial oxygen saturation, pulse rate, perfusion index, and total hemoglobin.

I wrote about this device a year ago after an abstract was presented at EAST. The final paper from the University of Arizona – Tucson has now been published,  and here are the updated factoids:

  • 525 patients were spot-checked, with a success rate of 86% 
  • Spot-check failures were due to nail polish or soot on the nails, sensor fit problems (only one size was available in the study), placement problems due to other imaging equipment, or patient agitation
  • 173 (38%) of patients had a Hgb <= 8
  • The mean difference between spot-check and blood draw results was only 0.3 g/dL (!)
  • Sensitivity was 95%, accuracy 76%

Bottom line: This is an interesting new tool for acute trauma care. The only downside that I see is that we may lose sight of the fact that hemoglobin values lag behind as an indicator of true blood volume in rapidly bleeding patients. We mustn’t be fooled into thinking that everything is fine just because a number is normal. There’s still room for common sense! And don’t start monitoring serial hemoglobins willy nilly in solid organ injury just because you can. You still don’t need it!

Related posts:

Reference: Transforming hemoglobin measurement in trauma patients: noninvasive spot check hemoglobin. J Am Col Surg 220(1):93-98, 2015.