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.
- 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.
- 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 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.