Short coagulation samples. Why does it happen? Why is it a problem?

Underfilled Coagulation Tubes

Coagulation tubes (blue top) are filled with a solution of sodium citrate which acts as an anticoagulant by chelating calcium. Calcium is essential for blood clotting. When coagulation tests are performed in the laboratory, calcium is added to allow clot formation. Citrate is used as an anticoagulant for clotting tests because it is has a low saturation level and its effects can be reversed by the calcium levels in the clotting reagent.

The tubes have been designed so the ratio of citrate to blood is enough to ensure that all the calcium in the sample has been chelated, thus preventing clotting of the sample, and accurate testing by the laboratory. The tubes have a blood fill tolerance of 10% of the level marked on the label (see picture left) and this is FDA approved. If samples are under-filled, the citrate to blood ratio is too high and therefore, depending on how short, the added calcium will take a time to reverse the citrate effect leading to increased clotting times.

The laboratory asked a few staff “guinea pigs”, one whom is on warfarin and two that are not, to have some coagulation samples taken with various levels of blood to see the effect on the results. Results for the staff not on warfarin ranged from borderline normal, where the clinician may be inclined to repeat or order further clotting or liver tests, to quite abnormal on samples ¾ full to ¼ full. The staff member on warfarin has a treatment range of 2-3, the normal result was 2.3 but with a sample only ¾ full the result rose to 2.8. When the half full sample was tested the result had increased to 3.4 which would have lead to a decrease in warfarin dose, running the risk of under anticoagulation and subsequent risk of thrombosis .

There are several reasons for under filled samples, which include poor veins, poor technique/inexperience and low vacuum in the sample tubes. The vacuum of all types of sample tubes will diminish over time and within the last few months of storage may not draw the correct amount of blood. This has been recognised by the manufacturer with regard to the citrate tubes and they have started to issue tubes with 9 months expiry.

Effect of Expiry Date on Fill Levels

Any tubes sent for analysis that have passed their expiry will not be accepted by the laboratory, especially citrate (blue) and oxalate (grey), due to the possibility of inaccurate analysis.

Many out of date bottles are returned to the pathology department therefore we ask users to order supplies only when needed and ensure good stock rotation to reduce wastage.

It is important blood results are correct and the laboratory aims to keep variables that can occur to a minimum through robust quality procedures. The laboratory has no control over the pre-analytical stage of the process which is probably the most important stage. The pre-analytical process covers the storage of the specimen tubes, taking of the sample and transport to the laboratory and any of these steps can influence the result.

More information on blood taking procedures or regarding the influences of the pre-analytical stages can be found at the Tube manufacturers web site

Coagulation Bottle Fill Tolerance Indication

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The laboratories at Heartlands Hospital, Good Hope Hospital and Solihull Hospital form part of the services provided by University Hospitals Birmingham and are UKAS (United Kingdom Accreditation Service) accredited to the ISO 15189:2012 standard. For a list of accredited tests and other information please visit the UKAS website using the following link:

  • Heartlands, Good Hope and Solihull Hospital pathology laboratories are a UKAS accredited medical laboratory No.8217
  • United Kingdom Health Security Agency laboratory is a UKAS accredited medical laboratory No.8213

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For further information contact Louise Fallon, Quality Manager, 0121 424 1235