I’ve had a few people ask me about this test today after I posted about it in my list of blood tests that I’ve had in the last year, so I figured I’d share some more information about it.
For starters, the test is officially known as the Dilute Russell’s Viper Venom Time (DRVVT). While it’s not a rare test per say, it’s not generally ordered as a part of routine blood work . Typically, a doctor will order this if they are looking for specific markers relating to certain blood clotting disorders, such as those in Antiphospholipid Syndrome (APS). Still, when I called for my test results the nurse who read my result had never even heard of this test and had no idea what the result meant – other than it was flagged as being abnormal (more about that later). Nevertheless, the test itself is really interesting (and slightly confusing as well).
Although my test was done in the laboratory, this video will give you a good demonstration of how and why viper venom is used to test the coagulation times of human blood.
If you have a phobia of snakes – DO NOT watch the following video
Pretty amazing, huh?
Also kind of scary to think about what happens to the body when an individual is bitten by such a venomous snake. Luckily, though, that wasn’t the case here.
About the Test:
According to the Practical Haemostasis Website, the “Russell’s viper venom [RVV] isolated from the snake Daboia russelii [orrussellii – contains a potent activator of factor X which in the presence of phospholipid [PL], prothrombin and calcium ions clots fibrinogen to fibrin. In individuals with a lupus anticoagulant [LA] the antibody binds to the phospholipid inhibiting the action of the RVV and prolonging the clotting time.
As the RVV directly activates factor X, the test is unaffected by deficiencies of factors XII, XI, IX or VIII. The DRVVT is frequently combined with a platelet neutralisation procedure to demonstrate the phospholipid specificity of the antibody” (para. 1).
One thing to note here is that the Lupus anticoagulant does NOT test for lupus – although the test may be positive for some individuals if they have both Lupus AND APS. It’s merely an immunoglobulin that is associated with an increased clotting time.
How It’s Used:
From The American Association for Clinical Chemistry
Lupus anticoagulant testing is a series of tests used to detect lupus anticoagulant (LA) in the blood. LA is an autoantibodyassociated with excess blood clot formation. LA testing may be used to help determine the cause of:
- An unexplained blood clot (thrombosis) in a vein or artery
- Recurrent miscarriages
- An unexplained prolonged PTT test (PTT measures the time it takes in seconds for a person’s blood sample to clot in a test tube after reagents are added.) LA testing helps determine whether a prolonged PTT is due to a specific inhibitor, such as an antibody against a specific coagulation factor, or to a nonspecific inhibitor like the lupus anticoagulant.
LA testing may also be used:
- Along with tests for cardiolipin antibody and anti-beta2-glycoprotein I to diagnose antiphospholipid syndrome (APS)
- Along with tests such as factor V Leiden or proteins C and S to help diagnose an excessive clotting disorder (thrombophilia)
- To determine whether the lupus anticoagulant is temporary (transient) or persistent
LA cannot be measured directly and there is no single test or standardized procedure to detect the presence of LA in the blood. A series of tests is used to confirm or rule out the autoantibody:
It is recommended that two tests be used to detect lupus anticoagulant. The most sensitive tests are dilute Russell viper venom test (DRVVT) and a LA-sensitive PTT (PTT-LA), one that uses low levels of phospholipid reagents. Follow-up testing is performed to confirm or exclude the presence of lupus anticoagulant. These may include:
Mixing study: an equal volume of patient plasma is mixed with “normal” pooled plasma and a PTT or DRVVT is performed on this mixture.
Correction/neutralization: an excess of phospholipids is added to the patient sample and a PTT-LA or DRVVT is performed. (When PTT-LA is measured, the assay is called a hexagonal phase phospholipid neutralization assay).
The Laboratory Method:
From Practical Haemostasis
Pooled normal plasma is mixed with diluted PL at 37°C. Diluted RVV and then calcium chloride are added and the clotting time is measured. The test is then repeated using patient plasma and the ratio of test:normal plasma is calculated.
Reagent Explanation Platelet poor Plasma A source of coagulation factors, particularly thrombin and fibrinogen. Dilute Russell’s Viper venom Typically this is diluted to give a clotting time in normal plasma of between 30-35s as this optimises sensitivity to antiphospholipid antibodies. Phospholipid To provide a surface for thrombin generation. This should be diluted to a sufficient degree that it becomes the rate limiting step and any inhibition by a LA prolongs coagulation. Calcium To initiate coagulation
My Test Results:
Although my test results were out of the normal limits, it is not considered a positive test because my result would need to be higher than normal range (slower clotting time) and mine was below the average (faster than the normal clotting time). Make sense? No idea if a low score has any clinical significance – haven’t found anything that even mentions it.
Below are some explanations about the meaning of the results and how to determine a positive test.
From The American Association for Clinical Chemistry
The results of the series of LA tests either lead toward or away from the likelihood of having LA. The laboratory report may be somewhat complicated, but it usually provides an interpretation of the results and states whether LA is present or absent. LA testing results, like those of other tests for clotting disorders, are difficult to interpret and are best evaluated by physicians with experience with excessive clotting disorders.
Although the initial tests performed for LA may vary, they usually begin with a PTT that is prolonged. A PTT that is normal (not prolonged) may mean that there is no LA present. However, the test may not be sensitive enough to detect LA and the LA-sensitive PTT (PTT-LA) may need to be done.
Additional details on LA testing results
Lupus anticoagulant testing is often done in conjunction with tests for cardiolipin antibody and anti-beta2-glycoprotein I antibodies to help diagnose antiphospholipid syndrome. The results are interpreted together, along with clinical criteria, in order to make a diagnosis.
Results of other tests that may be performed to help rule out other causes of a prolonged PTT include:
- If a thrombin time test is normal, then heparin contamination is excluded as a cause of prolonged PTT.
- If a fibrinogen test is normal, then it is likely that there is sufficient fibrinogen for clot formation.
Other tests that may be done to help confirm the diagnosis of a lupus anticoagulant include:
- Coagulation factor assays – these may be ordered to rule out factor deficiencies that may cause a prolonged PTT and bleeding episodes; a panel of factor assays may also help in detecting lupus anticoagulant.
- Complete blood count (CBC) – the CBC test includes a platelet count; mild to moderate thrombocytopenia (low platelet count) is often seen along with the lupus anticoagulant; moderate to severe thrombocytopenia may develop in patients receiving anticoagulant (heparin) therapy for lupus anticoagulant-associated thrombosis.
- Tissue thromboplastin inhibition test (rarely performed nowadays)
From Practical Haemostasis
If the DRVVT is not prolonged then the correction test (sometimes referred to as the neutralisation test) is not indicated.
DRVVT Interpretation DRVVT
Ratio [Test DRVVT/control DRVVT]
Reference Range: 29-42s
Reference Ratio: 0.9-1.05
Ratio [Test DRVVT/control DRVVT] >1.05 Possible LA
Exclude deficiencies of factors II, V, X, fibrinogen or another non-LA inhibitor
Prolonged DRVVT which corrects with normal plasma Clotting factor deficiency
[A weak LA can sometimes be masked in a 1:1 mix with normal plasma and some labs recommend a 1:4 mix [Normal plasma:Test plasma] to try and minimise this]
Prolonged DRVVT which corrects with PL Lupus anticoagulant
Calculating the percentage correction:
1. The percentage correction is calculated using the formula below where ‘+ PL’ is the DRVVT with additional phospholipid i.e. the neutralisation step.
2. The percentage correction of the ratio is calculated using the formula below where again ‘+ PL’ is the DRVVT with additional phospholipid. The final result is multiplied by 100 to convert it into a percentage:
This latter calculation is that currently recommended by the British Committee for Standardisation in Haematology [BCSH.]
In the following example:
Sample dRVVT Clotting Time [s] Ratios Patient Plasma 69.2 s [Patient Plasma]/[Reference Plasma] = 1.82 Reference Control [Normal] Plasma 37.9 s Patient Plasma + Phospholipid 39.5 s [Patient Plasma + Phospholipid]/Reference Control [Normal] Plasma + Phospholipid] = 1.21 Reference Control [Normal] Plasma + Phospholipid 32.5 s
From the data above: [1.82 – 1.21/1.82] x 100 = 33.5% correction. So in a patient in whom the dRVVT is prolonged as in this case [69.2s] the 33.5% correction is consistent with the presence of a lupus anticoagulant.
3. What constitutes a positive correction ratio? Most laboratories regard a correction of >10% with PL as being a positive test.
In individuals in whom a LA is identified, the test should be repeated in 12 weeks. It should also be remembered that not all tests including the DRVVT will identify all LAs and therefore, if the index of suspicion that a specific patient has a LA then other tests should be undertaken e.g. Silica Clotting Time [SCT]. Finally – the causes of a LA should be screened for e.g. ANA, drugs, viruses etc.
American Association for Clinical Chemistry (2014). Lupus Anticoagulant Testing. Retrieved from https://labtestsonline.org/understanding/analytes/lupus-anticoagulant/tab/test/
Practical Haemostasis (2016). Dilute Russell’s Viper Venom Time [DRVVT]. Retrieved on June 6, 2016, from http://www.practical-haemostasis.com/Thrombophilia%20Tests/APS/drvvt.html
Sports One Media (2012, July 12). Viper Venonom Turns Blood Into Jelly [Video File]. Retrieved from https://www.youtube.com/watch?v=PwT8vDzjCSQ&list=PLXGuqM-FQFlipPDC8hcLvG7CQ2F8divMo&oref=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DPwT8vDzjCSQ%26list%3DPLXGuqM-FQFlipPDC8hcLvG7CQ2F8divMo&has_verified=1