The Dilute Russell Viper Venom Time (DRVVT) screen is a specialized laboratory blood test used in coagulation studies to investigate unexplained blood clotting issues. It measures how quickly blood plasma clots after specific reagents are added, assessing a part of the coagulation cascade. The DRVVT screen is a functional assay designed to detect autoantibodies that interfere with the normal blood clotting process.
The Target Condition: Lupus Anticoagulant
The DRVVT screen is primarily employed to identify Lupus Anticoagulant (LA), a problematic autoantibody. LA is an antiphospholipid antibody mistakenly produced by the immune system that targets complexes of proteins and phospholipids. Phospholipids are fatty molecules that provide the necessary surface for clotting factors to assemble and initiate coagulation.
The name “Lupus Anticoagulant” is misleading because most people with the antibody do not have Lupus Erythematosus. Furthermore, despite its name, the antibody increases the risk of dangerous blood clots rather than preventing them—an effect known as the Lupus Anticoagulant paradox.
In the laboratory (in vitro), LA prolongs clotting time by binding to the phospholipids in the test reagent, preventing efficient clot formation. Inside the body (in vivo), this activity is associated with thrombosis, leading to conditions like deep vein thrombosis, stroke, and recurrent pregnancy loss. LA presence is a key laboratory criterion for diagnosing Antiphospholipid Syndrome (APS), a disorder characterized by excessive blood clotting.
The Two-Step Testing Mechanism
The DRVVT test is a functional assay that uses Russell’s viper venom (Daboia russelii) to initiate the clotting process. The venom contains an enzyme that directly activates clotting factor X, bypassing the complex steps of the intrinsic clotting pathway. This direct activation makes the DRVVT less susceptible to interference from deficiencies in other clotting factors.
The DRVVT is performed in two steps: a screening phase and a confirmation phase. The initial screening step uses a reagent with a dilute concentration of phospholipid, making the test sensitive to LA presence. If LA is present, it binds to the limited available phospholipid, significantly prolonging the time required for a clot to form.
If the screening time is prolonged, the confirmation or neutralization step is performed. This phase repeats the test using the same reagents, but with a large excess of phospholipid added to the mixture. The excess phospholipid neutralizes the LA, allowing clotting factors to work without inhibition. If the prolonged clotting time significantly shortens in this phase, it confirms that the inhibition was phospholipid-dependent, the hallmark of Lupus Anticoagulant.
Understanding DRVVT Results
The final interpretation of the DRVVT screen relies on calculating a ratio that compares the results of the two testing phases. This ratio, determined by dividing the screening test clotting time by the confirmation test clotting time, is the definitive measure used to establish LA presence. A normal DRVVT screen ratio is typically less than 1.20, indicating that LA is not detectable.
A positive result is suggested when the initial screening time is prolonged and the calculated ratio is 1.20 or greater. This elevation confirms the presence of a phospholipid-dependent inhibitor (LA) because the clotting time significantly shortened after the addition of excess phospholipid. Conversely, a prolonged screening time that does not significantly shorten suggests a cause other than LA, such as a clotting factor deficiency or interference from medication.
A prolonged DRVVT time can sometimes be caused by factors other than LA, including certain medications. Therefore, comprehensive interpretation requires considering the patient’s overall health and drug regimen. Laboratory reports typically express the result as positive, negative, or recommend retesting after a specified period.
Clinical Context and Next Steps
A doctor typically orders the DRVVT screen when a patient experiences unexplained thrombotic events, such as stroke or blood clots, or recurrent pregnancy complications. It is also requested when a routine coagulation test, like the Activated Partial Thromboplastin Time (APTT), is unexpectedly prolonged. The DRVVT is valued because its specific mechanism makes it reliable for LA detection compared to general clotting assays.
A positive DRVVT result is not a final diagnosis, as LA presence can be temporary due to acute infection or drug use. Clinical guidelines require that a positive LA test be confirmed by retesting the patient at least 12 weeks after the initial result. This retesting ensures that the LA is persistently present, which is required for a definitive diagnosis of Antiphospholipid Syndrome (APS).
Further testing for other antiphospholipid antibodies is necessary to complete the diagnostic picture for APS. These include tests for anti-cardiolipin antibodies and anti-beta-2 glycoprotein I antibodies. If LA presence is confirmed, management involves risk stratification and treatment with anticoagulant medications to reduce the risk of future blood clots.