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The page below is a sample from the LabCE course Minimizing Pre-Analytical Variability During Venipuncture, Urine Sample Collection, and Sample Processing. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Investigation: Severely Decreased Serum Calcium

This level of calcium is certainly unanticipated, as levels this low are not compatible with life. Calcium is very tightly regulated by the parathyroid to help maintain normal heartbeat and muscle function.
An investigation of a severely low serum calcium should include:
  1. Verification of the sample identity, including patient name, medical record number, and date of birth.
  2. Examination of the serum aliquot tube, if prepared, and the original collection tube to verify that the sample type is indeed serum, and not urine or other body fluid, in which the calcium may be much lower than serum.
  3. Seeking possible sources of chelation, such as EDTA, which binds calcium and renders it non-detectable by colorimetric methods.
  4. Analysis of the sample for potassium, which would be very highly elevated if the sample was contaminated with potassium EDTA.
In this case, the potassium level was run from the original sample tube, and the result was 8.8 mEq/L (reference interval 3.5 - 5.3 mEq/L). This combination of results strongly indicates contamination of the sample with potassium EDTA. It was subsequently determined that a new phlebotomist poured some whole blood from the lavender/purple-top tube for the CBC into the serum separator tube, which was a short draw. The calcium test was cancelled, and a redraw was requested. The phlebotomist received additional training as to the policy of not transferring blood from one tube to another.