Blue Top Tube

In clinical chemistry, it is a tube with sodium citrate as an anticoagulant, used to collect specimens for coagulation factor assays, fibrinogen, glucose-6-phosphate dehydrogenase, PTT, PT, thrombin time

also known as:  Sodium citrate tube

It’s used for routine coagulation testing.

Most coagulation procedures require a blue top citrated tube. When collection is made with a vacutainer needle, blood should be allowed to enter the tube until the vacuum is exhausted. A 4.5ml tube will be approximately two-thirds full. A proper ratio of blood to citrate is required for valid results.

The blue top tubes are calibrated to maintain the correct ratio of plasma to the sodium citrate anticoagulant based upon a hematocrit up to 55%. Therefore, patients with a hematocrit greater than 55% or less than 21% will not have correct ratio of plasma to citrate and the clotting studies may give false negative or false positive results.  Specimens must be delivered to the laboratory labeled with the patient’s name, unit number, date and time of collection as soon as possible after collection.

Coagulation Factors

Also known as: Factor assays; Blood clotting factors; Clotting factors [or by the individual factor number (Factor I, Factor II, etc.) or name (Fibrinogen, Prothrombin, etc.)

Coagulation factors are a group of proteins essential for blood clot formation. When a patient has an unexplained bleeding episode, one possible cause is a reduction in the level of a coagulation factor in their blood. Measuring the level of these factors can help a doctor determine the cause of the bleeding and the best treatment. Levels may also be measured if someone has a family history of bleeding.

In most cases, the level of a coagulation factor is determined by measuring the activity or function of the factor in blood. Activity assays can detect reduced levels of protein or proteins that don’t work properly (have reduced function). Rarely, the antigen level of a coagulation factor may be measured. Coagulation factor antigen tests can tell how much of the protein is present but not whether its function is normal.

When an injury occurs that results in bleeding, the coagulation system is activated and plugs the hole in the bleeding vessel with a clot while still keeping blood flowing through the vessel by preventing the clot from getting too large. The coagulation system consists of a series of proteins (coagulation factors) that activate in a step-by-step process called the coagulation cascade. The end result is the formation of insoluble fibrin threads that link together at the site of injury, along with aggregated cell fragments called platelets to form a stable blood clot. The clot prevents additional blood loss and remains in place until the injured area has healed. Blood clotting is dynamic; once a clot is formed other factors are activated that slow clotting and begin to dissolve the clot in a process called fibrinolysis. The clot is eventually removed as the injury site is healed. In normal healthy individuals, this balance between clot formation and removal ensures that bleeding does not become excessive and that clots are removed once they are no longer needed.

There are nine coagulation factor proteins that are routinely measured clinically (see table below). These factors are referred to by a name or Roman numeral or both in some cases. For example, coagulation factor II is also known as prothrombin. When one or more of these factors are missing, produced in too small a quantity, or not functioning correctly, they can cause excessive bleeding.

coagulation factor Other common name
I Fibrinogen
II Prothrombin
VIII Antihemophilic factor A
IX Antihemophilic factor B

How is the sample collected for testing?

A blood sample is drawn from a vein in the arm.


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