Why order hemoglobin w/ crit?

[bbpiano1]

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Everyone is always concerned about the H&H, but why do you need both values? The hematocrit is usually 3x the hemoglobin, so why not just put an order in for hemoglobin levels? The attendings didn't really have an answer.

 

[LadyWolverine]

For the same reason that, for a pt with anemia + neuro sx you always order B12/folate. Or the same reason that the ED orders CEx3 and blood cx on everyone who walks in the door.

 

[QofQuimica]

Everyone is always concerned about the H&H, but why do you need both values? The hematocrit is usually 3x the hemoglobin, so why not just put an order in for hemoglobin levels? The attendings didn't really have an answer.

My understanding is that hemoglobin is a measured value while hematocrit is usually calculated based on RBC count and RBC volume. Not that this answer helps very much, because it begs the question of why bother calculating hematocrit. 😛

 

[edmadison]

QofQ is right about the calculation. However, labs can still do old-fashioned manual spun 'crits. I've done this several times with NICU babies because getting blood can be a b*tch and often clots in the bullet tube before it can be analyzed. It takes just a few drops and a capillary tube to do a spun 'crit.

Ed

 

[yaah]

Modern hematology instruments measure and calculate multiple parameters during each run. Hct and Hgb are two of them. You can't really order one without the other unless you do tests the old fashioned way, which is inefficient.

Also, when MCV or hemoglobin are abnormal, the 3:1 ratio may not be true. The ratio only applies to normal erythrocytes. For all practical purposes though having both doesn't necessarily give you a whole lot of extra info. In some senses having the 3:1 ratio serves as a nice control that your "normal" values for both are truly normal. If not, there may be an issue that just measuring one of them might not detect.

 

[dienekes88]

Can't you also have hypochromic anemia with a normal-ish crit?

 

[Winged Scapula]

Because in the hospitals I've worked the full CBC or H&H are actually cheaper than a single test. Get a list sometime of the charges. Very interesting.

 

[lapelirroja]

My understanding is that hemoglobin is a measured value while hematocrit is usually calculated based on RBC count and RBC volume. Not that this answer helps very much, because it begs the question of why bother calculating hematocrit. 😛

Yep, the machine lyses the RBCs and measures Hgb spectrophotometrically. Lots of the other things are calcs (indicies). Basic CBC includes it all, use the data you want, don't bother with the rest.

 

[apannier]

remember also that the hematocrit is volume dependent, so a normal hemoglobin with a high crit could show in a patient that is volume contracted

 

[anon-y-mouse]

remember also that the hematocrit is volume dependent, so a normal hemoglobin with a high crit could show in a patient that is volume contracted

If we get a value for hemoglobin in mg/dl, then wouldn't that value also change in a person who is volume contracted? i.e. hemoconcentration... I would think that both values would change.

Also, yaah, what are some examples of H/H non-3:1 mismatch? can you think of particular disease states where this would be a useful clue?

 

[yaah]

You're right, hemoglobin is also volume dependent (dL is the demominator).

As for your question, not really any specific answers. Abnormal red cells such as those in hemoglobinopathies (thalassemias) or myelodysplasia can have abnormal ratios. But there are probably other clues for those (in other indices) that are more helpful. Increased reticulocytes also would probably alter it.