85610

Prothrombin Time (PT);

Prothrombin time (PT) is a fundamental laboratory test used to evaluate the extrinsic and common pathways of the coagulation cascade. This assay measures the time, in seconds, for plasma to clot after the addition of tissue factor (thromboplastin) and calcium. The process begins with a blood draw, typically using a light-blue top tube containing sodium citrate, which acts as an anticoagulant by binding calcium. In the laboratory, the sample is centrifuged to isolate platelet-poor plasma. The test specifically assesses the functional activity of clotting factors I (fibrinogen), II (prothrombin), V, VII, and X. Factor VII is unique to the extrinsic pathway, making the PT highly sensitive to deficiencies or inhibitors of this specific factor. One of the most common clinical applications of 85610 is the monitoring of patients on oral anticoagulant therapy, primarily warfarin (Coumadin). Because warfarin inhibits the synthesis of vitamin K-dependent factors (II, VII, IX, and X), it prolongs the PT. To ensure consistency across different laboratories and reagents, the PT result is frequently converted into the International Normalized Ratio (INR). The INR is calculated using the formula: INR = (Patient PT / Mean Normal PT) raised to the power of the International Sensitivity Index (ISI). Beyond medication monitoring, the PT is essential for diagnosing bleeding disorders, assessing liver synthetic function (as the liver produces most coagulation factors), and screening for vitamin K deficiency. It is also a standard component of preoperative testing to evaluate a patient's risk of intraoperative or postoperative hemorrhage. A prolonged PT may indicate liver disease, disseminated intravascular coagulation (DIC), or the presence of circulating anticoagulants or inhibitors.

Clinical Indications

  • Monitoring of warfarin (Coumadin) anticoagulant therapy
  • Preoperative screening for potential bleeding risks
  • Evaluation of unexplained bleeding or bruising
  • Assessment of liver synthetic function and disease severity
  • Screening for Vitamin K deficiency
  • Diagnosis of disseminated intravascular coagulation (DIC)
  • Evaluation of suspected factor deficiencies in the extrinsic or common pathways

Procedure Steps

  1. Collection of venous blood into a 3.2% sodium citrate (light-blue top) tube.
  2. Centrifugation of the specimen at a high speed to obtain platelet-poor plasma.
  3. Incubation of the plasma at 37 degrees Celsius to mimic body temperature.
  4. Addition of a specific volume of tissue thromboplastin reagent combined with calcium chloride.
  5. Measurement of the elapsed time from the addition of the reagent until the first appearance of a fibrin clot, usually detected by optical or mechanical means.
  6. Calculation of the International Normalized Ratio (INR) using the laboratory's specific ISI value.

Coding Guidelines

  • Report 85610 for the measurement of prothrombin time.
  • The calculation and reporting of the International Normalized Ratio (INR) are considered part of 85610 and should not be billed separately.
  • If the test is performed in a CLIA-waived laboratory setting using an approved device, the modifier QW (CLIA waived test) must be appended.
  • Do not use 85610 for home-based PT/INR monitoring services; refer to codes 93792 or 93793 for those specific management services.
  • When performed with a Partial Thromboplastin Time (PTT), report both 85610 and 85730 as they evaluate different parts of the coagulation pathway.

Associated ICD-10 Codes