Specimen Handling for a Stat Potassium
A phlebotomist collects a stat potassium level from a patient in the emergency department. The patient has an IV line in the left arm infusing lactated Ringer's solution. The phlebotomist draws the blood from the left arm, just above the IV site, using a 21-gauge butterfly needle and a 5 mL serum separator tube (SST). The tourniquet is left on for 2 minutes. The sample is sent to the lab immediately, but the result shows a potassium of 6.8 mmol/L, which is critically high. The patient's previous potassium was 4.2 mmol/L. The lab calls to report the critical value.
1. List at least three factors in this collection that could cause a falsely elevated potassium result.
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- Drawing above an IV site: Drawing blood from the same arm as an IV infusion can contaminate the sample with the IV fluid. Lactated Ringer's contains potassium (4 mEq/L), which can falsely elevate potassium.
- Prolonged tourniquet time: A tourniquet left on for 2 minutes can cause hemoconcentration and hemolysis, releasing potassium from cells.
- Hemolysis: Use of a butterfly needle with a small gauge (21G is acceptable but could cause hemolysis if the vein is fragile), or excessive suction, can cause hemolysis, which falsely elevates potassium.
- Serum separator tube (SST): SST contains a gel that can be contaminated with potassium from the gel, but this is less common. However, if the sample was not centrifuged promptly, potassium can leak from cells.
- Delayed processing: If the sample was not centrifuged within 2 hours, potassium can leak from red blood cells.
2. What actions should the phlebotomist take to correct the problem and prevent recurrence?
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The phlebotomist should:
- Immediately recollect the sample from the opposite arm (right arm) to avoid IV contamination.
- Use a suitable vein, preferably not above an IV site, and apply the tourniquet for no more than 1 minute.
- Use a needle size appropriate for the vein; a 21G butterfly is acceptable, but for potassium testing, a larger needle (e.g., 20G) may reduce hemolysis.
- Collect the sample in a tube appropriate for potassium testing; serum separator tubes (SST) are acceptable, but plasma tubes (lithium heparin) are preferred for potassium because they avoid clotting and reduce hemolysis risk.
- Ensure the sample is centrifuged promptly (within 2 hours) and separated from cells.
- Document the recollection and notify the provider of the critical value.