True regarding prothrombin time measurement?
Major screening tests of coagulation 

In all of these tests, blood is collected with a calcium-chelating anticoagulant (usually citrate) and then centrifuged to remove the erythrocytes, leukocyte and platelets.
The remaining platelet-poor plasma (PPP) serves as the substrate for these tests.
Depending upon the test, the phospholipid is added to the PPP to substitute for platelets and an activating agent is added.
The PPP is then recalcified and the time needed for a clot to form is measured.
The sample to be kept at room temperature if it is to be used for prothrombin time, Lupus anticoagulant or factor VII assays and to be kept at 4° C for other assays.
Once blood is drawn for testing, the tests should be completed within a time frame of 30 minutes to 2 hours. 

Measurement of prothrombin time

The PT measures the function of the extrinsic and common coagulation pathway. It is commonly used to monitor oral anticoagulant therapy.
Method:- A source of phospholipid and tissue thromboplastin (rabbit or human brain extract) is added to platelet poor plasma, and Ca2+ is added to initiate coagulation. The time for the appearance of fibrin strands (PT) is measured and is compared with the control.
Normal PT is 11- 13 seconds.

Measurement of Partial thromboplastin time (PTT) and activated partial thromboplastin time (APTT or aPTT)

The PTT and APTT are used to measure the function of intrinsic and common coagulation pathway. PTT and APTT are used to monitor heparin therapy.
PTT method:- Phospholipid is added to platelet poor plasma taken from blood that is anticoagulated with citrate. Clotting is then initiated by the addition of Ca2+. The time to the formation of first fibrin strands is PTT.
APTT method:- The APTT is performed in the same manner as PTT except for the addition of an activating agent such as kaolin or ellagic acid before recalcification.
Normal PTT is 28-35 seconds. 
Measurement of thrombin time
It tests the conversion of fibrinogen to fibrin, i.e., it screens thrombin-fibrinogen reaction.
Method:- Thrombin is added to the patient's platelet poor plasma and the time to fibrin strand formation is measured
Normal TT is 15- 20 seconds.
Other screening tests
These are:-


Fibrinogen assay
Platelet function tests ( bleeding time)
Screening for factor XIII
Other individual factor assay
Platelet rich plasma is required
Activate with kaolin
Should be measured within 2 hours
Immediate refrigeration to preserve coagulation factor viability