PROTHROMBIN TIME
(PT)

Sample type: A blood sample (citrated)

Uses:

Evaluation of liver function reflecting abnormalities in factors VII, II, X, and V (but not VIII).

To monitor long-term oral anticoagulant therapy with coumarin and indanedione derivatives.

PT is prolonged >PTT, and more consistently

Precautions:

Some medications, supplements, food and alcohol may affect test results.

Examples of medications that may affect PT test results include:

• Some antibiotics.

• Barbiturates.

• Oral contraceptives.

• Hormone replacement therapy.

Interfering factor:

• Diet: ingestion of excessive green, leafy vegetables increases the body’s absorption of vitamin K, which promote blood clotting.

• Alcoholism or excessive alcohol ingestion prolong PT level

• Prolonged storage of plasma at 4c –activate factor VII and shorten PT

• Diarrhea and vomiting decrease PT because of dehydration

Pre-analytical errors:

Partially clotted specimens due to poor mixture with the anticoagulant (3.2 sodium citrate, as offered by manufacturers, blue top vacuum tubes).

Corrective action: Good mixing of the sample with the anticoagulant

Post analytical errors:

• Check instrument settings

• Assure that all preventive maintenance has been done

• Carry out troubleshooting functions called for in the SOP

• Call for equipment support

Corrective action:

• Assure that the prescribed preventive maintenance for the instrument is performed

Reference range:

PT: 11-13 seconds