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Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

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PROTHROMBIN TIME (PT)

Item Value
Approval req'd? no
Available Stat? Yes
Test code PT
Performed by? Hematology
Sendout? no
Price range $
In House Availability Stat or routine, 24 hours/7days
Principle This one-stage test involves adding a thromboplastin reagent (containing tissue extract and CaCl2) to citrated plasma and timing it until a clot is formed.
Interpretation The Prothrombin Time is prolonged by an abnormality or deficiency in any of the factors required in the extrinsic clotting system. It is less sensitive to an isolated Factor II deficiency; only the most severe forms will prolong the Prothrombin Time.

The laboratory also reports the International Normalized Ratio (INR) on each Prothrombin Time. The INR is equal to the patient's PT divided by the geometric mean of the current reference range, raised to the power of the ISI (International Sensitivity Index) that is unique to each lot of thromboplastin reagent used. Thus,

INR = (Patient's PT / Geo Mean of PT Reference Range)ISI = (PT Ratio)ISI
Geometric Mean of PT Reference Range with current reagent = 13.1 (effective date 9/30/16)
ISI of current reagent = 1.28 (effective date 9/30/16)


Therapeutic INR range: 2.0 - 3.0

Therapeutic INR range for patients with mechanical prosthetic valves: 2.5 - 3.5

CLINICAL SIGNIFICANCE: The PT measures Factors I, II, V, VII and X. Isolated prolongation of the PT (normal APTT) usually suggests acquired F VII deficiency (early liver disease, vitamin K deficiency or warfarin therapy).
Container type blue top tube
Amount to Collect full tube (2.7 mL; 1.8 mL pediatric tube)
Collection Instructions See Coagulation Procedures for instructions on collecting coagulation specimens.
Sample type Citrated plasma
Special instructions Indicate Use Of Anticoagulant Drugs On Requisition Slip.
Normal range (Adult): < 14.6 sec. (Current range; changes with new lot of reagents.)

Pediatric reference ranges for PT have not been established at SFGH. In general, healthy term neonates have 15 – 30% longer PT values, while children from 1 year old to adolescence have slightly lower PT values (up to 10% shorter) compared to adults.
Critical values PT > 45.6, INR > 4.9
Synonyms INR;Protime;PT;
Stability 24 hours
Additional information Screening for suspected coagulation and bleeding disorders; monitoring Coumadin therapy.
References 1. Triplett, D. and Harms, C.S. Procedures for the Coagulation Laboratory. Chicago, IL: Am. Soc. of Clin. Path., 1982.

2. Hirsh, J. Et al. Oral Anticoagulants. Mechanism of Action, Clinical Effectiveness, and Optimal therapeutic Range. Chest 108: 227-S - 246-S (1995).

3. Amelung AMAX CS-190 Operator's Manual, 3/97.
CPT coding 85610
Last Updated 10/2/2016 5:15:38 PM
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