Published on May 11, 2016
Why are Lab Reference Ranges Different?
“How much of a difference in counts can you expect to see if you have blood drawn and processed at different labs?” In this MPN Ask the Expert segment, Dr. Naveen Pemmaraju answers this question from viewer, Cindy. Dr. Pemmaraju discusses the variation in labs and offers tips to track your results, following trends versus the results of a single test.
The Ask the Expert series is sponsored through an educational grant to the Patient Empowerment Network from Incyte Corporation.
Transcript | Do Blood Test Results Differ When Processed at Different Labs?
Please remember the opinions expressed on Patient Power are not necessarily the views of our sponsors, contributors, partners or Patient Power. Our discussions are not a substitute for seeking medical advice or care from your own doctor. That’s how you’ll get care that’s most appropriate for you.
Comparing Results from Different Labs
Cindy sent in this question. Cindy wants to know, “How much of a difference in counts can you expect to see if you have blood drawn and processed at different labs?”
What an important question. Absolutely, this happens, and I’m surprised a lot of times at how few providers and maybe even patients and families are aware of this. So this is a great opportunity to talk about it. Yes, there is a difference from lab to lab, period. It has nothing to do with academic versus community or one state to another. It just has to do with time points, variations and calibration of instruments.
Now, sometimes if it’s a minor difference by hundredths of a point, okay fine. But sometimes this can be a significant difference all the way up to a point or higher—meaning if we’re talking about hemoglobin and into the tens for platelets. Well, I think I can think of three important aspects of this. One is to recognize that there are variances and so that minor variances are part of what we do, and so accuracy and precision comes in.
If you have the same set of blood drawn at the same place, okay then you have some reliable time point measurement for that. But what I tell my patients—and as you know a lot of my patients do get blood drawn on the outside and here—you need to have a graph and a time point to measure over time. And so that’s the key thing, and you want to measure lab to lab at the same lab if you can.
So that’s number one. Number two is when you get a lab that is completely different from the one you’ve been getting at the other place, as always you have to take that seriously. Yes, it could be that there was an instrument variation, but don’t assume that first. Go ahead and assume that it’s a real difference and either get it re-checked at the original place or if needed, get it checked emergently so you can have the tiebreaker there. So sure, every once in a while there is an off-calibrated instrument.
The third factor is if it’s possible in a non-emergency setting what we try to do is try not to make decisions based on a single lab in general, whether it’s at the original place or the second place. And so for this question that we were asked, yes it happens. It happens very frequently. We expect it to happen. But if there’s a big discrepancy, you need to repeat the lab either at another place or at the original place and make decisions on a trend rather than on one time point.
Do I Need to Go to the Same Lab for Follow Up Tests?
I have one follow-up question about testing. Is it important for the patient who has sequential lab tests being followed—I’m an example of that—try to go to the same lab, but also same time of day? Is it related to your diet at all and how much water you’ve had or anything like that that would affect it?
All of the above, yes. In general, most of the things that you just mentioned should only cause a minor variance and over time. So over years, we expect—you and I would—that all of these, they all cancel each other out, which they do. I’ll give two examples of where that actually could matter. You mentioned both of them: hydration and diet. There are some medications which are exquisitely sensitive to the type of diet, and the one that we all know is the drug Coumadin or warfarin, the blood thinner. It’s exquisitely sensitive to patients who have a difference in their vitamin K levels, for example, spinach and greens.
That’s a great question that you just asked. A lot of our patients are on Coumadin. I’m glad you asked that, so yes diet can matter. Patients who are not on Coumadin, possibly it could matter, but generally I would say that there’s not a big variance.
Hydration is actually very interesting. Sure, if somebody is overhydrated, it could possibly cause a little bit more of a diluted specimen, and that’s important for us because of hematocrit. Again, I would say for the vast majority of patients and the vast majority of times the time of day should not matter. But these are two kinds of circumstances where if a lab is a little bit off, it might be worth repeating. So anyway, those are the two that I’ve seen in the clinic that can make a difference.