Low Ionized calcium on blood gas samples

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An RN in our Trauma unit had three different blood gas samples in a row from three different patients come back with absurdly low ionized calcium results (0.13 mmol/L, 0.14 mmol/L, 0.33 mmol/L). All other blood gas results from the samples were normal. I repeated the last sample on a different analyzer and obtained the same result. I then ran some heparinized whole blood from LiHep vacutainers on both of the analyzers and the ionized calcium results were normal. Subsequent samples run on the analyzers did not repeat the issue. I did not observe her collect the samples, but she demonstrated that she collected the samples from a venipuncture into blood gas syringe, and no other blood draws were collected. I feel like this has to be a sample collection issue, but I am at a loss as to the cause.  Any thoughts?

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Exposure to air increases the pH which causes proteins to bind to calcium resulting in low ionized calciums.

My guess is there has to be a chelating agent involved here that bound the calcium.  Most common culprit is EDTA; I know in our main lab we see this when staff will collect in an EDTA vacutainer and "pour" it into a heparinized tube when they realize they drew the wrong tube and don't want to recollect.  Usually that's accompanied by an elevated K+ (because it's K3EDTA), but you indicated all the other results were normal?

Hi James,
EDTA contamination was my first thought too, but the potassiums were not elevated (one was actually below our normal range), and she says she collected directly from the vein and no other tubes collected. I feel there has to be some contamination somewhere, I just don't know where or how.

Very bizarre.  I can't imagine anything other than a binding agent causing such a significant depression in results.  Has to be something in the collection equipment, but as you said, what. 

And wow, I just noticed you said it was three different patients.  Has to be collection equipment.

Heparin does bind Calcium.  Could the blood gas syringes have had too much heparin in them?

Hi Liz,
As part of my investigation I did draw up blood from the vacutainer using their syringes and the result was normal, however I did not get the syringe from the same drawer the nurse used, so maybe the supplies she used were contaminated somehow. I will have to check that possibility out.

But electrolyte-balanced heparin (typically what's in a true blood gas syringe) should not be affecting electrolytes, hence the name.  Any chance a rogue syringe/container was used first?

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Lois Schultz
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