Sources of Error

The following are common sources of error in phlebotomy and laboratory testing.

  1. Diet influences certain tests which require fasting prior to specimen collection.
  2. Tourniquets left on for more than 2 minutes or vigorous hand exercises can elevate potassium and lactic acid levels, and falsely decrease blood pH.
  3. Hemolysis causes falsely increased levels of acid phosphatase, bilirubin, CPK, LDH, magnesium, potassium, etc.
  4. Serum in prolonged contact with the clot can result in changes in glucose, iron, LDH, potassium, insulin etc.
  5. Sodium citrate (blue-topped) tubes for PT, PTT or special coagulation tests involve a dilution factor with the anticoagulant which is figured into the final results, so the tubes must be filled to be adequate.  Do not overfill by forcing more blood into the tube than the vacuum draws in. Under filled tubes will be rejected and must be redrawn. When drawing with a butterfly, do not collect the blue-top tube first, as the amount of air in the line will cause inadequate volume to be collected.