CMP from finger stick

[newkidnurse]

I had a 5yo patient the other night who was known to be a really difficult stick. So for her morning labs (just a CMP), I asked the IV team to draw them. The IV nurse was unsuccessful after 1st attempt, so then decided to get the patient's labs via finger stick.

Results came back (not hemolyzed, yay!) and the patient's K was slightly elevated--I think it was maybe 5.1 or 5.2? Shortly after, the resident changed the patient's maintenance IVFs to fluids without 20KCl.

One of the RNs I work with asked me if the resident was aware that her CMP came from a finger stick. When I asked why this mattered, she told me that she had heard that CMPs from finger sticks often have falsely elevated Ks. This RN wasn't sure why, and I haven't been able to figure out why this would be the case (if it is true).

Have you heard this? If so, can you please explain why?? Thank you!!

[RNNPICU, BSN, RN]

K is an intracellular ion. When you are squeezing the finger to get a finger stick for labs, the potential for K to be elevated is becuase you are squuezing the blood out and "crushing" the cells causing K to leak out into extracellular fluid...

t it could be that the K might have been slightly high

[newkidnurse]

K is an intracellular ion. When you are squeezing the finger to get a finger stick for labs, the potential for K to be elevated is becuase you are squuezing the blood out and "crushing" the cells causing K to leak out into extracellular fluid...

t it could be that the K might have been slightly high

Thanks for the response! That reasoning makes sense as far as why finger sticks are usually/often hemolyzed... and I appreciate the explanation as to why the presence of hemolysis = increased K.

BUT, in a non-hemolyzed finger stick sample, I'm not totally convinced that this would still be the case? Maybe just a small amount of 'cell crushing' (so less leaking than 'true hemolysis' but more leaking than a normal venous sample) will elevate the K but not be clinically labeled as hemolysis?

**I assume most labs put in a comment if the sample was hemolyzed (our lab does)

[newkidnurse]

Sorry to keep on this... But if the lab does NOT specify the presence of hemolysis, can I assume the finger stick K will still be slightly higher than a venous sample taken at the same time? Thanks!

[RNsRWe, ASN, RN]

VERY interesting! I have never heard of this practice, and it intrigues me. Would be interested in knowing the details of the elevated K+ when a sample is obtained this way. Honestly didn't think there'd be enough blood obtained for a valid sample!

Learned something new today. Beautiful thing :)

[TiffyRN, BSN, PhD]

In the NICU, we don't do fingersticks but almost all routine labs are by heelstick, so same concept/concerns. Hemolysis is our constant companion. We don't even call K levels unless they are >6.5 (unless we know it came from a line draw).

Now, I can't tell you how many times I have sent down a heelstick that I know was a hard draw, kid just won't bleed and the poor kid's heel is already very bruised from previous draws. Lab calls a little later with the "panic value" of K=6.7. They are severely worried because "there is no hemolysis!" Sorry, lab dude, I'm the one that drew that lab, I know how hard it was to get that miniscule 0.4ml sample, I know what that heel looked like and I know what my kid looks like (no EKG changes, good UOP). I don't care if you swear there is no hemolysis, there just is! We re-draw from an arterial stick and the hyperkalemia is cured, the K is now 4.4.

Magic.

Yes, if your provider is going to make changes to the KCl based on a fingerstick lab and this is not routine for your environment, I would let them know because even in the absence of lab-designated hemolysis, finger or heelsticks can render falsely elevated K levels.

[TiffyRN, BSN, PhD]

VERY interesting! I have never heard of this practice, and it intrigues me. Would be interested in knowing the details of the elevated K+ when a sample is obtained this way. Honestly didn't think there'd be enough blood obtained for a valid sample!

Learned something new today. Beautiful thing :)

One of the biggest shocks to me when I went into NICU was how much lab can be derived from extremely small samples. Lab previously had me buffaloed into believing they couldn't do lytes (4-6 values) with less than 4mls of blood. Absolute minimum for a CBC would be 2mls.

I get to NICU and find out that lytes could be done on 0.4ml, CMP off of 0.8ml (depending on the HCT, those really chronically anemic kids probably only need 0.6ml).

Blood gas; don't need 1ml, 0.2ml will work just fine in a capillary tube.

CBC; that needs all of 0.25ml.

I honestly felt betrayed.

Send outs are a problem, but in-house labs don't really need all that much volume.

[newkidnurse]

Yes, if your provider is going to make changes to the KCl based on a fingerstick lab and this is not routine for your environment, I would let them know because even in the absence of lab-designated hemolysis, finger or heelsticks can render falsely elevated K levels.

Tiffy- thanks! This is exactly what I was looking for... Any idea WHY this happens ("no hemolysis" but elevated K)?? Maybe there is a certain threshold of hemolysis that can occur and alter values but will be undetected by labs... So interesting! Thank you!

[newkidnurse]

Came across this interesting article which includes many reasons for falsely elevated K (also termed pseudohyperkalemia).

Asirvatham JR, Moses V, Bjornson L. Errors in potassium measurement: A laboratory perspective for the clinician. North Am J Med Sci [serial online] 2013 [cited 2015 Jan 2];5:255-9. Available from: Errors in potassium measurement: A laboratory perspective for the clinician Asirvatham JR, Moses V, Bjornson L - North Am J Med Sci

Really interesting because it says pneumatic tube systems have been known to cause this to occur as well, and my hospital JUST started using one of these in Nov!

PS- All of this is why I love being a nurse. There is always so much we get to think about and consider :)