Blood Collection Factors Affecting Results

Over / Under Filled Tubes

  • It is very important that all tubes with anticoagulants / 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, a short draw (3 mL) EDTA tube should be used.

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

  • 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 and renin, 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.