Protocol - Prothrombin Time

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This protocol provides an overview of the prothrombin time assay according to the local laboratory. Some examples include LabCorp test 005199 or Quest Diagnostics test 8847. The prothrombin time assay measures the extrinsic, or tissue factor, pathway of the coagulation system and is used to monitor liver biosynthetic function and success of oral anticoagulant therapy. Because there are many comparable assays and instruments for measuring PT, the protocol also provides basic guidelines to aid comparability among different studies.

Specific Instructions

The Sickle Cell Disease Curative Therapies Working Group recommends that investigators collect and report prothrombin time but not international normalized ratio (INR). INR has been validated only in patients on coumadin therapy. It is, however, used extensively by hepatologists for assessment of liver disease severity in liver transplantation and is a major determinant for prioritization of organ allocation through the Model for Endstage Liver Disease (MELD) score.




Specimen Requirements and Storage Instructions

The prothrombin time assay can be performed on whole blood or plasma. 4.5 milliliters of sample should be collected in a blue-top tube with 3.2% buffered sodium citrate. Specimens should be discarded if the tube is less than 90% full. The sample should be inverted six or more times to ensure mixing of the blood with the anticoagulant. The sample is stable for 24 hours at room temperature. For longer storage, specimens can be centrifuged and then frozen.

Summary of the Prothrombin Time Assay

The prothrombin time assay measures the extrinsic or tissue factor pathway of the coagulation system. The prothrombin time assay is performed on platelet-poor plasma to which 3.2% sodium citrate has been added to chelate calcium and prevent coagulation. Thromboplastin (tissue factor plus phospholipid) and calcium chloride are added to the sample to begin the assay. The time (in seconds) for a fibrin clot to form is measured by optical or mechanical methods. Normal prothrombin test values range from 10 to 13 seconds.

Prothrombin Time Reference Ranges



0 to 30 days

9.8−13.2 seconds

1 to 6 months

9.8−12.6 seconds

7 months to 1 year

9.8−12.2 seconds

2 to 17 years

9.9–12.1 seconds

≥18 years

9.1−12.0 seconds

The Sickle Cell Disease Curative Therapies Working Group notes that there are several different assays and instruments that are appropriate to measure the prothrombin time. Once an assay is chosen for a particular study, the Working Group recommends that no changes in the protocol be made over the course of the study. To aid comparability, the Working Group recommends that the investigator record the name of the assay, the make and manufacturer of equipment used and the repeatability and coefficients of variation for the assay.

Personnel and Training Required

A phlebotomist is required to draw the blood sample.

Equipment Needs

Laboratory with the equipment needed to perform the prothrombin time assay

Requirement CategoryRequired
Major equipment No
Specialized training No
Specialized requirements for biospecimen collection No
Average time of greater than 15 minutes in an unaffected individual No
Mode of Administration



Infant, Toddler, Child, Adolescent, Adult


All ages

Selection Rationale

The prothrombin time (PT) test is widely available, standardized methodology for collecting, processing, and storing blood, performing the prothrombin test, and reporting results.



Derived Variables


Process and Review

Not applicable.

Protocol Name from Source

Prothrombin Time (PT), CPT 85610


Labcorp. (n.d.). Prothrombin Time (PT). Retrieved February 23, 2021, from https://www.labcorp.com/tests/005199/prothrombin-time-pt

Quest Diagnostics. Prothrombin Time with INR. Retrieved July 16th, 2021, from Prothrombin Time with INR | Test Detail | Quest Diagnostics

General References

Adcock, D. M., Kressin, D. C., & Marlar, R. A. (1997). Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. American Journal of Clinical Pathology, 107(1), 105–110.

Clinical and Laboratory Standards Institute. (2008). One-stage prothrombin time (PT) test and activated partial thromboplastin time (APTT) test; approved guideline—second edition (CLSI Document H47-A2).

Feld, J. J., Kato, G. J., Koh, C., Shields, T., Hildesheim, M., Kleiner, D. E., Taylor, J. G., 6th, Sandler, N. G., Douek, D., Haynes-Williams, V., Nichols, J. S., Hoofnagle, J. H., Jake Liang, T., Gladwin, M. T., & Heller, T. (2015). Liver injury is associated with mortality in sickle cell disease. Alimentary Pharmacology & Therapeutics, 42(7), 912–921.

Mohamed, E. A., Elgari, M. M., Babker, A. M., & Waggiallah, H. A. (2020). Comparative study of hypercoagulability change in steady state and during vaso-occlusive crisis among Sudanese patients living with sickle cell disease. African Health Sciences, 20(1), 392–396.

Reneke, J., Etzell, J., Leslie, S., Ng, V. L., & Gottfried, E. L. (1998). Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. American Journal of Clinical Pathology, 109(6), 754–757.

Sang Medicine. (n.d.). Prothrombin Time [PT]. Retrieved April 12, 2021, from https://www.practical-haemostasis.com/Screening%20Tests/pt.html

Protocol ID


Export Variables
Variable Name Variable IDVariable DescriptiondbGaP Mapping
PX850101030000 What was the time for a fibrin clot to form? N/A
PX850101020000 Was the sample inverted at least six times more
to ensure mixing of the blood with the anticoagulant before performing the prothombin time assay? show less
PX850101010000 What type of sample was used for the more
prothrombin time assay? show less
Curative Therapies
Measure Name

Prothrombin Time

Release Date

August 16, 2021


Prothrombin is a blood clotting factor produced by the liver.


Prothrombin time measures the conversion of prothrombin to thrombin, which requires the presence of clotting factors made in the liver (e.g., factors II, V, VII, and X) and reflects liver synthetic function. Prolonged prothrombin time is a marker for acutely decreased liver synthetic function and can indicate progression to cirrhosis or impending liver failure. Liver dysfunction is common in a variety of diseases and can impact candidacy for and morbidity from curative therapies.


Liver, coagulation, clotting

Measure Protocols
Protocol ID Protocol Name
850101 Prothrombin Time

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