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Test Code LAB320 Prothrombin Time (PT)

Performing Laboratory

St. Louis Children's Hospital - Coagulation

Specimen Requirements

Patient Preparation: None

Container/Tube  
Preferred: Light blue-top (Citrate) tube (1.8 or 2.7 mL)
Acceptable: Blue-cap (9Cl/1.3) microtainer tube. Call lab at 314-454-4268 if microtainer tube required
Collection Volume: Tube should be full

(Full is to marking on tube label. MINIMUM tube volume for collection is to marking on tube label but may go above line) 

Submission Container/Tube: If specimen is coming from outside facility and do not have a STAT spin centrifuge to process, centrifuge and transfer all plasma into a plastic vial, then centrifuge plasma again to get platelet-poor plasma. Aliquot plasma into plastic vial leaving 0.25 mL in the bottom of centrifuged vial. Send to lab STAT
Special Instructions: 

1.

Draw a discard tube prior to light-blue top tube.

2.

Tube(s) should remain stoppered.

3.

Immediately invert tube/microtainer® gently 8-10 times after collection to ensure proper mixing of blood and anticoagulant (this will avoid specimen clotting). Forward promptly at ambient temperature only.

4.

DO NOT underfill tube(s)

5.

SPECIMEN MAY NOT BE COLLECTED BY CAPILLARY METHOD

6.

If the patient has a known hematocrit >55%, contact the lab at 454-4268 for collection instructions and a specially adjusted collection tube(s).

 

Sample Rejection

Vacutainer®(s) that are under-filled.

Clotted specimens.

Grossly hemolyzed specimens.

Mislabeled or unlabeled specimens.

Logistics

Test Availability: Monday-Sunday
Cut-off time: None; performed as received
Turnaround Time: Same day; 1 hours from receipt in performing lab

Reference Values

Reference Interval: (all ages)

PT: 10.2-13.5 seconds                                  
INR: 0.90-1.20 
The recommended Therapeutic range for Oral anticoagulants: INR of 2.0-3.0 on standard dose

Critical Value: 

PT None established
INR (all ages):  >5.0                                                                  

Critical test results will automatically be called back initially and every 72 hours thereafter

Methodology

Absorbance measurement, clot signature formation

Limitations

  • NA

Additional Information

FOR SLCH CORE LABORATORY USE ONLY

Lab Processing Instructions: GIve specimen to coagulation bench to be spun in STAT Spin centrifuge for 3 minutes @ 8500 rpm
Processed Volume: Blood level should be to line on label (may be over)
Analyte Stability:

Specimen Type Temperature Time
Whole Blood (unprocessed)  Ambient 24 hours
Plasma  (processed off cells)    Refrigerated (preferred) 24 hours
  Ambient  24 hours
  Frozen  (-20c) 1 month     

Test Classification and CPT Code

85610

LOINC

PT: 5902-2

INR: 6301-6

Last Reviewed

07/2025