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Coagulation Screen


Coagulation Screen
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Reference Range:

Activated Partial Thromboplastin Time (APTT):
Frimley Park Hospital = 23 - 35 seconds
Royal Surrey County Hospital = 0.8 - 1.2

Prothrombin Time (PT):
Frimley Park Hospital = 11 - 15.3 seconds
Royal Surrey County Hosital = 0.9 - 1.18

Fibrinogen: 1.9 - 4.5g/l

Turnaround Time:

Urgent: 1 hr / In Patient: 2 hrs / GP: 24 hrs


If patients are on anticoagulants the type of anticoagulant should be stated clearly in the request.
All relevant clinical details must be provided with the request.


The process of coagulation occurs in three phases: the vascular platelet phase, which assures primary hemostasis; activation of the coagulation cascade, which assures formation of the clot; and activation of a series of control mechanisms, which stop propagation of the clot and limit activation of the coagulation cascade to the region of endothelial rupture. Coagulation screen consists of in vitro tests—the activated partial thromboplastin time (aPTT) and, prothrombin time (PT), which measure the time elapsed from activation of the coagulation cascade at different points to the generation of fibrin.


For coagulation screen, 9 volumes of blood added to 1 volume of sodium citrate solution. This ratio of anticoagulant to blood is critical as osmotic effects and changes in free calcium ion concentrations affect the coagulation results. Obtaining blood from an in-dwelling line or catheter is a potential source of error due to likely heaprin contamination, therefore sampling from those is unacceptable.

Vessels and Containers:

Blue Citrate 4.5 ml
For paediatric patients use paediatric blue Citrate container

Sample Requirements:

Underfilled samples will invalidate results
Haemolysis will invalidate results
All medication must be written on the request form
APTT samples must be tested within 6 hours


Heparin and Warfarin will prolong the results of a coagulation screen
Haemolysis shorten the clotting time
Lipaemic samples - clot detection is difficult

How result is reported:

Results are reported as per reference range, if the sample quality is acceptable.
If the sample quality is not acceptable (due to overfilled or underfilled or haemolysed or clotted samples) then the test request will be cancelled and the reason for cancellation is reported.

Important Information:

Citrate Samples which are more than 4hrs old will not be processed by the laboratory.


Clotting Screen

Transportation and Handling:

Samples which are to be transported by hand or local transport should be dispatched in clearly sealed individual plastic bags within 4 hours of collection.
If it is sent from longer distances, the person sending the specimen must ensure that the appropriate specimen container is used and samples must be spun (following protocol)and plasma should be separated and frozen to -70*C prior to transport. Contact Laboratory if advice required. .

Time limits for requesting additional examinations:

All add on tests should be requested within 4hrs from the time sample taken.