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Blood Collection Factors Affecting Results

Over or Under Filled Tubes

  • It is very important that all tubes with anticoagulants or additives are filled to the correct blood to additive ratio in order to obtain accurate results.  For example, tubes should be filled to within 90% of the maximum volume of the tube.

CBC Testing

  • Fully filled tubes are always preferred.  In certain situations where obtaining a full tube is difficult or unlikely, there must be a minimum of 1.5 mL of blood in a 4 mL EDTA tube in order to obtain accurate CBC results.
  • If unable to obtain even this amount of sample, use a short draw (3 mL) EDTA tube.

ESR Testing

  • There must be a minimum of 2 mL of blood in a 4 mL EDTA tube for ESR testing.  If CBC and ESR are ordered together, there must be a minimum of 3.5 mL of blood in one EDTA tube.


  • Hemolysis (the bursting of red blood cells) gives serum a red colour and can adversely affect results.
  • Causes of hemolysis include:  use of a very small bore needle, forcing blood from a syringe into an evacuated tube, leaving a tourniquet on too long, "shaking" tubes as opposed to mixing by gentle inversion.

Improper Use of Tourniquet

  • The tourniquet should not be left on for longer than one minute. 
  • There are certain tests, such as lactate, in which a tourniquet must not be used at all or the results can be elevated.
  • Leaving a tourniquet on a patient for too long can cause incorrect blood test results (formation of petechiae can also result).

Improper Mixing of Tubes

  • All tubes, even those without additive, must be mixed a certain number of times using gentle and full inversions immediately post collection:
    • 3 to 4 times for blue top sodium citrate tubes
    • 5 times for SST and red top tubes
    • 8 to 10 times for all other tubes
  • Improper mixing of tubes can cause inaccurate results and/or require specimen recollection.

Collection Factors Affecting Results: Safety and Notes

When presented with a choice of collecting a PST tube or an SST tube, collect a PST tube.

  • PST tubes can be processed immediately upon receipt in the lab, reducing turnaround times especially for stat test results.
  • SST tubes have to clot for 30 minutes before the specimens can be processed.