The Mysteries of the Coagulation Collection Tube and Anticoagulant


Errors in sample collection and processing are costly although often preventable. There are three main sources of errors: pre-analytical, analytical and post-analytical. Of the most important of these is the pre-analytical phase, accounting for up to 75% of all errors in some studies. The reasons for this are complex however they are usually related to blood sample collection, handling, storage, transport or processing. Despite this, the solutions are usually simple.

Getting it right is vital for the patient. Analysing a suboptimal sample will potentially yield a false result and falsely inform clinical decisions. The patient may require additional venipuncture where the sample is rejected, imparting distress on them and the staff involved.

The anticoagulant

Coagulation samples are at particular risk of pre-analytical error. To adequately perform a coagulation screen*, the sample should be anticoagulated immediately after collection, to maintain coagulation factors in the form of zymogens and to prevent activation and clotting. The sample should also remain activatable when performing certain coagulation tests. Sodium citrate does this job and hence is the anticoagulant of choice of coagulation samples. Serum samples trigger the clotting cascade and are therefore unsuitable. Anticoagulants like EDTA and lithium-heparin irreversibly inhibit coagulation and are also unsuitable.

The collection tube

The coagulation sample tube is identified by a light blue coloured cap. The tube is usually made of plastic and even siliconized glass. Whatever the material used, it should be of adequate resistance to damage from transportation, storage at low temperatures and centrifugation.

Why the citrate concentration is important

This is most pertinent to the accuracy of the International Normalised Ratio (INR), the measure of anticoagulant activity for patients treated with warfarin and other vitamin K antagonists. The variable nature of the thromboplastin used in different laboratories can be overcome by calibrating against an International WHO reference thromboplastin (International Reference Preparation or IRP) to assign them an International Sensitivity Index (IS)I.

The ISI of a thromboplastin may be affected by the strength of citrate anticoagulant used for sample collection during thromboplastin calibration. The higher the citrate concentration used, the higher the INR. The International Council for Standardisation in Haematology (ICSH) recommends using a trisodium citrate concentration of 105-109 mmol/L (3.1%-3.2%). The WHO recommends using 109 mmol/L trisodium citrate for ISI/INR work or a mixture of trisodium citrate and citric acid if the total citrate plus citric acid concentration is 109 mmol/L and the pH is ≥5. Higher concentrations, usually 129 mmol/L (3.8%) trisodium citrate, should be avoided.

It is important that a standard citrate concentration is used between patient blood samples and calibration samples used to assign an ISI of a particular thromboplastin.

If you are considering changing the type of blood collection tube used, it is important to check the alternatives against existing tubes.

It is important to standardise the approach to these factors. A key factor in achieving this and reducing errors, a written policy on acceptable blood collection systems/tubes for testing performed is mandatory for any laboratory.

* a coagulation screen constitutes an activated partial thromboplastin time (APTT) and prothrombin time (PT). Some laboratories will also report a fibrinogen concentration, most often by the Clauss method.
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  1. Bonini, P., et al. (2002). Errors in laboratory medicine. Clinical chemistry, 48(5), 691-698
  2. Kitchen, S., et al. (2021). International Council for Standardisation in Haematology (ICSH) recommendations for collection of blood samples for coagulation testing. International Journal of Laboratory Hematology
  3. Practical heamostasis: International Sensitivity Index [ISI] & The International Normalised Ratio [INR].
  4. WHO Expert Committee on Biological Standardization. 2013; Guidelines for thromboplastins and plasma used to control oral anticoagulant therapy. WHO Technical Report series. No 979, Annex 6, Geneva: World Health Organization.

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