The Order of draw is the process in which sample tubes are collected. Order of draw is a system used when drawing more than 1 tube or the sequence in which tubes are filled from a syringe. Order of draw is designed to make sure that the least amount of test contamination occurs should additive carryover happen. It is essential that this part of your phlebotomy training is well understood.
Order of draw ensures that instances of carryover can be minimized by making certain that specimen tubes are filled from the bottom up and by making sure that the contents of the tube do not touch the needle during the draw or transfer of blood into the new containers. It also helps to identify what could have gone wrong at a particular stage, if problems occur.
For Evacuated Tube System (ETS) collection and when filling tubes from a syringe, the following order is recommended by the Clinical and Laboratory Standards Institute (CLSI):
- Sterile tube (blood culture)
- Blue-top coagulation tube
- Serum tube with or without clot activator, with or without gel
- Heparin tube with or without gel plasma separator
- EDTA tube
- Glycolytic inhibitor tube
If tubes are filled in the wrong order, the risk of cross contamination exists. If there is cross contamination, it can adversely affect the testing of the specimen.
Cross contamination presents the following risks to testing:
- Additive carryover- this is when the syringe used to fill a tube makes contact with the blood/additive mixture of another tube and transports even a minute portion of this mixture into the next tube that is filled.
- Tissue thromboplastin- Thromboplastin is a protein that is present in tissue fluid. It helps to initiate blood coagulation. Therefore it can contaminate coagulation tests. Thromboplastin is no longer considered to be a substantial danger to the results of prothrombin time (PT) or partial thromboplastin time (PTT) tests.
However, it can complicate the results of other coagulation tests. Because of this, it is necessary to draw a small amount of blood into another non-additive collection tube before drawing a coagulation test specimen. This should be done any time that a PT or PTT sample is not the first or only container collected.
Please be advised that a discardable tube must be drawn to safeguard the critical 9:1 blood-to-additive ratio of a coagulation tube that is the first or only tube collected using a butterfly because air in the tubing moves blood inside of the tube.
Microorganisms – A microorganism is a single or multicellular organism that can be transferred from one tube to the next by use of syringe. Microorganisms are detected by blood culture and so test tubes must be spotlessly clean to avoid sample contamination. Blood culture tubes are sterile. They are collected first in the order of draw process in order to make sure that the sterility of the tube is not compromised. If the tubes are not sterile, false positives may occur and this can delay or result in incorrect patient treatment.
It should be noted, however, that different institutions may use varying sequences for order of draw. It is always advisable to first consult institutional policy before performing order of draw to minimize the chance of procedural mistakes.
Carryover or cross contamination
Carryover/cross contamination is the transfer of material from one tube into another tube by use of syringe. This happens when blood in an additive tube makes contact with a needle during ETS blood collection or when blood is transferred from a syringe into ETS tubes. Basically, blood that has made contact with the needle or is in the syringe can end up inside of the next tube that is drawn or filled. Should this happenin your career or during your training, the blood from the previous tube can affect the tests that will be performed on the specimen contained within the contaminated tube. This makes the test of the blood sample unreliable.
- EDTA (ethylenediaminetetraacetate) is the source of most carryover/ contamination problems in blood testing.
- Heparin naturally occurs in the blood and as a result, causes the least risk of cross contamination problems.
There are a number of other contaminating additives that can adversely affect a number of blood tests including:
- Sodium fluoride