What is PT Ratio?
The PT ratio is simply the patient's prothrombin time (PT) divided by a normal control PT, without any correction factor—it is NOT the same as INR and should not be confused with it. 1
Definition and Calculation
- PT ratio = Patient PT / Normal Control PT (or Mean Normal PT), representing a simple mathematical ratio without standardization 1
- The PT ratio was historically introduced in 1941 by Karl Link and colleagues as "the ratio of the logarithm of clotting times" to measure impaired coagulation in rabbits fed sweet clover hay 2
- This differs fundamentally from the INR, which uses the formula: INR = (Patient PT / Mean Normal PT)^ISI, where ISI is the International Sensitivity Index that corrects for reagent variability 3, 1
Critical Clinical Distinction
The PT ratio lacks the ISI correction factor that makes the INR standardized across different laboratories and reagents. 1 This is a common source of confusion in clinical practice.
Key Differences:
- PT ratio: Not standardized, varies between laboratories using different thromboplastin reagents 1
- INR: Standardized value designed specifically for warfarin monitoring, accounts for reagent sensitivity differences 2, 3
Historical Context and Evolution
- The PT ratio was first used clinically in the 1940s, with early case reports suggesting a PT ratio of 1.5 to 3 provided sufficient anticoagulation with dicoumarol 2
- PT results were not interchangeable between laboratories due to varying thromboplastin sensitivities, leading to excess bleeding in some patients and breakthrough clotting in others 2
- This variability necessitated the development of the INR system in the 1980s to standardize warfarin monitoring 3
Clinical Implications and Pitfalls
Never use PT ratio as a substitute for INR in warfarin monitoring—the lack of standardization makes it unreliable for therapeutic decision-making across different laboratory settings. 1
Common Errors to Avoid:
- Assuming PT ratio and INR are interchangeable terms 1
- Using PT ratio for cross-laboratory comparison of anticoagulation status 2
- Reporting PT ratio when INR is clinically indicated for VKA monitoring 3