If you order an ESR … will you know how to interpret it?

The question came the other day from Carol Baker, MD, of Texas Children’s Hospital, while doing rounds.

Somebody had told her that the erythrocyte sedimentation rate (ESR, or “sed rate”) results in a patient who received a blood transfusion would be difficult to interpret.

Have you heard something like that before?, she asked.

No, but I would be glad to check it for us. And here is what I found:

I started with Wikipedia, but it didn’t help this time. Then I turned to my old physiology book, Best & Taylor. Although my Spanish 12th edition is from 1993, I treasure many kind memories with this book. This didn’t help either. Nelson’s Pediatrics? ESR is scattered throughout the book and not expanded under a single heading as to address this question.

For these kind of topics, I like the American Academy of Family Physicians website. In a well-written article, Dr. Malcom Brigden explains that: “The test measures the distance that erythrocytes have fallen after one hour in a vertical column of anticoagulated blood under the influence of gravity.” (ergo, its units: mm/hr)

“Any condition that elevates fibrinogen (eg, pregnancy, diabetes mellitus, end-stage renal failure, heart disease, collagen vascular diseases, malignancy) may also elevate the ESR. Anemia and macrocytosis increase the ESR. In anemia, with the hematocrit reduced, the velocity of the upward flow of plasma is altered so that red blood cell aggregates fall faster. Macrocytic red cells with a smaller surface-to-volume ratio also settle more rapidly. A decreased ESR is associated with a number of blood diseases in which red blood cells have an irregular or smaller shape that causes slower settling.”

And so on. But if you have a minute, go on and check the Table 2 (Factors that influence ESR). Worth reading.

Going back to our conundrum, comparing results of the ESR before and after the transfusion may be greatly confounded. I would say that it told a valid statement: you may want to take the results with a grain of salt, or — probably better — order a different test (C-reactive protein?) to begin with, given the circumstances.

That would be enough for just a lab test.