4.29: Potassium

The head nurse of a cardiac care unit (CCU) calls the central chemistry laboratory about a low potassium result on a recently drawn plasma sample. The potassium level was measured in a small laboratory near the CCU. The CCU’s head nurse complains to the central chemistry laboratory’s supervisor that not only is this potassium result too low, but that a problem of “low” potassium results from the CCU laboratory has been noticed for the past week. The CCU’s results are “low” when compared to the results obtained from the central laboratory.

Potassium is measured in the CCU laboratory by an analyzer that uses a nondilutional ion selective electrode and whole blood, plasma, or serum as a sample. The central laboratory uses a dilutional ion selective electrode system for measuring potassium in serum or plasma.

QUESTION

What steps can the laboratory supervisor take immediately to validate the CCU’s measurement of potassium on the samples from the CCU?

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The following potassium results are reported back to the supervisor:

Specimen	CCU	Central Lab	Origin of Sample
1	3.8*	3.7	plasma from CCU sample
2	3.6*	3.7	plasma from CCU sample
3	4.1	4.1*	serum from central lab
4	3.7*	3.6	plasma from CCU sample
5	4.0	4.0*	serum from central lab

The starred (*) results are essentially equal to those obtained upon initial analysis of the whole blood or serum samples.

The laboratory supervisor takes these results and meets with the CCU nurse. The supervisor explains that there does not appear to be an analytical bias between the two laboratories; each instrument measures essentially the same result for each sample. Instead there appears to be a sampling problem - the potassium concentrations in the plasma (whole blood) samples is approximately 10% to 11% less than the potassium concentrations the serum samples.

QUESTION

What kind of phlebotomy error can lead to this discrepancy between the plasma and serum results?

Questions to Consider

1. Are there any sources of interference that might affect one potassium method but not another?
2. Are there any sample requirements that might affect the potassium results?
3. Is the magnitude of the difference between the plasma and serum potassium results to be expected?
4. What assumptions are usually made when phlebotomy tubes for plasma or whole blood are used?

Answer

1. The laboratory supervisor asks a medical technologist to take the following steps:
   • retrieve the most recent samples from the CCU for this patient including samples sent to the central laboratory
   • verify the patient’s name and identification code on the samples
   • note if there is anything unusual about the samples (lipemia, hemolysis, icteria, etc.)
   • perform a potassium analysis on all samples in both the CCU and central laboratories

The supervisor should ask a technologist to verify that the patient’s name and hospital identification number are on each tube to ensure that there was no sample mix-up. The plasma fractions from the whole blood samples and recent serum samples from this patient should be re-analyzed for potassium in both the CCU and central laboratories. This will help determine if an analytical bias exists between the two laboratories.

2. The most likely explanation for the significantly lower plasma levels for potassium is an underfilling of the phlebotomy tubes (p.71). If a 3.5 mL green-top tube containing 50 \(\mu\)L of heparin solution (see Table 3-2, p.71) is used, and the tube is only filled with 1.5 mL of blood, the dilution error would be 3.2% instead of the 1.4% that would exist if the tube were filled to capacity. The error for a blood potassium level of 4.0 mmol/L would be -0.130 mmol/L vs. -0.056 mmol/L, respectively. When this dilutional error is added onto the normal bias between plasma and serum potassium values, the total error becomes clinically obvious. As soon as the CCU’s phlebotomy tubes were properly filled, this bias disappeared. Obviously, if the phlebotmy tubes did not contain anticoagulant in a liquid form (that is, in a dried form instead), this problem would not have arisen.

Answers to Questions to Consider

1. One interference that would affect one method but not another is the effect of extreme hypertriglyceridemia (See Sodium and Potassium Method on CD-ROM), which would not affect the non-dilution method but might severely affect the dilutional method, depending upon how severe was the hypertriglyceridemia.
2. Serum, plasma, and whole blood samples can be used for the measurement of potassium (See Sodium and Potassium Method on CD-ROM). Sodium heparin is usually the anticoagulant of choice for whole blood and plasma samples. Ammonium heparin is unacceptable because the ammonium ions cause a positive interference with potassium ISEs (See Sodium and Potassium Method on CD-ROM).
3. No. There usually is a slight positive bias of 0.2 to 0.3 mmol/L between serum vs plasma because of the release of potassium from blood cells during the clotting process (pp.70, 73, and see Sodium and Potassium on CD-ROM). That is, plasma samples tend to run about 5% lower than a corresponding serum sample. The differences observed in this case are much greater than might normally be expected.
4. The assumption made when using phlebotomy tubes containing an anticoagulant are: 1) the tubes are filled to stated capacity, and 2) the filled tubes will be sufficiently mixed to ensure even dispersion of the anticoagulant in the sample.