Introduction

Prothrombin time (PT) is one of several blood tests routinely used in clinical practice to evaluate the coagulation status of patients. More specifically, PT is used to evaluate the extrinsic and common pathways of coagulation, which would detect deficiencies of factors II, V, VII, and X, and low fibrinogen concentrations.[1][2] PT measures the time, in seconds, for plasma to clot after adding thromboplastin, (a mixture of tissue factor, calcium, and phospholipid) to a patient's plasma sample.[1] Many different preparations of thromboplastin reagents are available which can give different PT results even when using the same plasma. Due to this variability, the World Health Organization (WHO) introduced the international normalized ratio (INR) and has become the standard reporting format for PT results.[3][4] The INR represents the ratio of the patient's PT divided by a control PT value obtained by using an international reference thromboplastin reagent developed by the WHO.[1][3]

Specimen Collection

Standard laboratory coagulation-based testing has traditionally been used to obtain measurements of PT to ensure reliable results. However, due to the high turnaround time of up to 90 minutes, point-of-care (POC) devices, with a turn-around time of approximately 5 minutes, are becoming more desirable. POC devices are of great value in the emergency and operating room settings where clinical diagnosis and intervention are time-sensitive.[5][6] With increased prescribing of vitamin K-antagonists (VKAs) like warfarin, point-of-care devices have also been more convenient for patients and general practitioners to monitor medication effectiveness. With point-of-care devices, monitoring anticoagulation therapy can take place at thrombosis centers, primary care provider offices, and even by the patients themselves.[4] Although point-of-care devices have been shown to underestimate hemostatic abnormality, point-of-care devices are generally reliable in non-emergency settings.[7] Coagulation tests must be performed using plasma samples and not serum as clotting factors get removed in serum preparations. Standard percutaneous phlebotomy is the recommended method used to collect venous blood samples. However, blood samples may also be obtained from indwelling intravenous lines when necessary.[8]

Procedures

Phlebotomists collect venous blood samples in plastic tubes with a light blue top that contains 3.2% sodium citrate.[9] Sodium citrate serves to chelate the calcium in the blood sample and prevents the activation of the coagulation cascade.[10] This chelation keeps the blood sample in stasis until ready to be tested. Tube filling must be within 90 percent of the full collection volume with blood to sodium citrate ratio of 9 to 1.[7] The tube is then gently inverted a few times to mix the sodium citrate solution with the blood. The tube should not be shaken to avoid hemolysis that would lead to inaccurate results. Once the blood sample is ready to be tested, calcium chloride is then added to restore the calcium required for coagulation activation.[7] Clot formation can then be detected mechanically or optically depending on the instrumentation used.[10]

Indications

Indications for obtaining PT are:

• Monitoring vitamin K-antagonists (VKA) such as warfarin is the most common indication for PT
• Evaluation of unexplained bleeding
• Diagnosing disseminated intravascular coagulation (DIC)
• Obtaining baseline value before initiating anticoagulation therapy
• Assessment of liver synthesis function and to calculate the model for end-stage liver diseases (MELD) score in liver disease

Potential Diagnosis

Causes for a prolonged PT include:

• Liver disease
  • Liver disease or liver dysfunction leads to a decreased production of most coagulation factors. A decreased production of coagulation factors leads to prolonged PT and physical manifestations that can include petechiae and easy bruising.[4]
• Vitamin K deficiency
  • Vitamin K is a necessary component in factors II, VII, IX, and X. A deficiency in vitamin K will lead to a decrease in these factors and prolong PT. Potential causes that can lead to decreased vitamin K levels include malnutrition, prolonged antibiotic use, and impairments with fat absorption.[9]
• Factor deficiency
  • Inherited diseases that lead to a decreased production of factors II, V, IX, and X will lead to a prolonged PT.
• Disseminated Intravascular Coagulation (DIC)
  • DIC causes a system-wide activation of coagulation, depleting available coagulation factors leading to an increase in PT.
• VKA therapy
  • VKA therapy inhibits factors II, VII, IX, and X and causes a prolonged PT.
• Antiphospholipid antibodies
  • Antiphospholipid antibody syndrome (APS) characteristically presents with recurrent thromboembolic events and/or pregnancy loss along with detected antiphospholipid antibodies (APA).[11][12] APA causes an increased conversion of prothrombin to thrombin in vivo, leading to an overall decrease in prothrombin. Low prothrombin levels can lead to an increased PT result.

Normal and Critical Findings

The reference ranges for PT vary by laboratory since different facilities use reagents or instruments. However, in most laboratories, the normal range for PT is 10 to 13 seconds.[10] The normal INR for a healthy individual is 1.1 or below, and the therapeutic range for most patients on VKAs is an INR of 2.0 to 3.0.[4] An increased PT/INR for patients on VKAs may suggest a super-therapeutic level and will require medication dose adjustments to prevent bleeding.

Interfering Factors

• Polycythemia with a hematocrit greater than 55%[10]
  • Elevated hematocrit greater than 55% leads to a decrease in plasma of the blood sample, thereby reducing the coagulating factors available. The sodium citrate levels must be readjusted to prevent artificially prolonged PT measurements, to account for this decreased plasma
• Underfilled tubes
  • Similar to polycythemia, underfilled tubes also will lead to an artificially prolonged PT measurement.
• Samples obtained from indwelling catheters may suffer contamination as these lines often require a flush with heparin or other solutions that would artificially prolong coagulation times[8]
• Anticoagulants
  • All direct acting anticoagulants prolong PT[13]
    • Argatroban
    • Dabigatran
    • Rivaroxaban
    • Apixaban
    • Edoxaban
• Storage and temperature
  • Blood samples for PT testing are only acceptable if stored for less than 24 hours at either room temperature or 4 degrees Celsius[1]
  • Prolonged cold storage at 4 degrees Celsius or lower can activate Factor VII, which can lead to shortened PT results[10]
• High lipid levels
  • Patients with hypercholesterolemia or hypertriglyceridemia have a shorter PT measurement due to more elevated fibrinogen and factor VII levels[14]

Complications

• Standard percutaneous phlebotomy to obtain blood samples can cause localized pain, bleeding, and bruising.
• A decreased PT/INR suggests[7][1][7]:
  • Increased intake of supplements that contain vitamin K
  • High intake of vitamin K-rich foods
  • Fasting may reduce factors II, VII, and X, subsequently increasing PT

Patient Safety and Education

As the use of VKAs increases, it is vital to educate patients on the importance of routine monitoring of PT/INR. Proper monitoring will allow for medication adjustments and the prevention of adverse events. If patients are self-monitoring with POC devices, sufficient education and training are necessary for the patient and/or family members who will assist the patient. The cognitive capacity of patients must also be evaluated to ensure the proper use of POC devices.[4]

Clinical Significance

PT, along with INR, are important measurements to monitor patient coagulation status, especially patients who are on VKAs. However, although PT/INR is useful in monitoring coagulation status, they are rarely used alone. PT/INR use is typically in conjunction with activated partial thromboplastin time (aPTT), which evaluates the intrinsic and common pathways of coagulation. PT/INR and aPTT results together can help in diagnosing various hematologic disorders.