UCSF Navigation Bar

UCSF Departments of Pathology & Laboratory Medicine Home Page

Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

Internal Resources

Click here for more information about laboratory operations and procedures

If you have additional questions regarding this test, please call: 415-353-1667

Search Our Test Menu

Prothrombin Time (Complete View)

Item Value
Test name Prothrombin Time
Test Update Information
Approval req'd? No
Available Stat? Yes
Utilization Guidelines
Test code PT
Test group Prothrombin time
Performed by Parnassus, Mission Bay & Mt. Zion Hematology
In House Availability Test available 24 hours per day 7 days per week
Method Mechanical clot detection
Patient Preparation
Collection Instructions 1. Check the expiration date on the label of the blue top vacutainer before drawing the patient

2. For blood collection in a sodium citrate blue top, the tube must be filled to above the Minimum Fill Indicator on the tube. It is crucial to wait and allow the tube to stop filling before removing it from the needle.

3. With use of a butterfly needle, draw about 1 cc using a separate blue top to remove air from tubing, discard the first tube and then draw a second blue top tube filled to the full extent of the vacuum.

4. Tubes should not be filled past the Maximum Fill dashed line by either using a syringe or removing the tube cap.

For patients with Hct's ≥ 55% please contact Hematology (For Parnassus patients call 3-1747, for Mission Bay patients call 6-0194) to obtain blue top tubes with adjusted citrate volumes in order to maintain the proper citrate to plasma ratio for coagulation studies.
Container type Blue (2.7 mL) or Lt. Blue (1.8 mL) top filled to full extent of vacuum
Amount to Collect
Blue top 2.7 mL blood
Lt. Blue top 1.8 mL blood
Sample type Citrated plasma
Preferred volume 1.5 mL plasma
Min. Volume 1 mL plasma
UCSF Rejection Criteria Samples collected in outdated blue top vacutainer.

Over-filled or under-filled samples may be rejected.
Processing notes
Ref Lab Rejection Criteria
Units seconds
Normal range
PT 11.8 – 15.8 seconds
INR 0.9 – 1.3

The PT of full-term normal infants does not appear to differ significantly from adults, although factor levels are somewhat lower in the first six months of life (Andrew, M et al. Development of the human coagulation system in the full-term infant. Blood. 70:165-172).
Critical value INR ≥ 5.0
Synonyms PT; coumadin; warfarin; INR
Stability Unopened, uncentrifuged specimens are stable for 24 hours at room temperature.
Turn around times STAT 1 hour, Routine 4 hours
Additional information Current reagents permit the reporting of results up to 100 seconds; if no clot is detected by that time the result will be reported as > 100 seconds.

The International Normalized Ratio (INR) will be routinely reported in addition to the Prothrombin Time in seconds. The reagents, and therefore the reference range in seconds, differ among the various UC affiliated hospitals. However, the INR adjusts for the inherent variability of tissue thromboplastins employed in the assay.

Note: when the prothrombin time is just above the upper limit of the normal range, calculation of the INR can result in a normal INR. This reflects the rounding of decimals utilized in the INR calculation.

The INR is the parameter of choice in monitoring the adequacy of warfarin anti-coagulation, and should in theory remain stable. The appropriate therapeutic range will vary with the disease and treatment intensity desired. The UCSF Hematology Consultation service generally recommends as therapeutic range for warfarin: (a) INR of 2.5-3.5 for most mechanical valves or recurrent systemic embolism, (b) INR of 2.0-3.0 for most other indications. Recommendations for warfarin therapy and monitoring are available through the UCSF Hematology consultation services: for adults, pager 443-4276, for pediatrics pager 443-6966.

The Prothrombin Time contains a reagent that neutralizes heparin up to 2 U/mL.

Per in-house study done 06/2017, the sensitivity of the PT for detecting factor deficiencies is as follows:

Factor PT prolongs when
Factor II < 31%
Factor V < 51%
Factor VII < 41%
Factor X < 39%

In regard to samples drawn through heparinized lines: (i) the PT reagent neutralizes low levels of heparin and (ii) additional heparin neutralization can cause a reduction in coagulation factor levels. For these reasons, heparin neutralization is not recommended for PT measurements.
CPT coding 85610
LOINC code 5902-2
LDT or Mod FDA?
Include for internal use?
Last Updated 7/31/2017 5:59:56 PM
Entry Number 800
Image caption
Lab Procedure Link
UCSF home page UCSF home page About UCSF Search UCSF UCSF Medical Center