From David Summers: I have a question regarding the following lab results and all clinical symptoms which seem to represent systemic lupus erythematosus (SLE):
- PTT LA 52 seconds, reference limit 40 seconds
- DRVVT screen 44 seconds, reference limit 42 sec
- Antithrombin antigen 39 mg/dL, reference interval 18–33 mg/dL
- Protein C activity 13%, reference interval 70–180%
- There is a negative anti-nuclear antibody (ANA) test.
Would this person be considered SLE positive?
Hi, Mr. Summers, and thank you for your question. The quick answer is no, based on the negative ANA test result, the person doesn’t have SLE.
Your question highlights a problem that plagues laboratory medicine. We do a poor job of naming our laboratory tests and medical disorders. SLE is a chronic inflammatory disease of connective tissue, part of a family that includes rheumatoid arthritis, scleroderma, and Sjogren’s syndrome. SLE is suspected based on signs and symptoms and confirmed using the ANA test profile.
Conversely, lupus anticoagulant (LA) is plasma antibody that reacts with phospholipid-bound proteins, especially a plasma protein called β-2-glycoprotein I. Though LAs are detected in a little less than 50% of SLE sufferers, they are also found in other conditions or may arise separately from any primary condition. SLE is rare, however LAs are found in 1–2% of the general population. Most LAs are transient, but chronic LAs are associated with strokes, transient ischemic attacks, acute myocardial infarction (heart attacks), peripheral artery disease, venous thromboembolic (VTE) disease (blood clotting in leg veins and in the lungs), and recurrent spontaneous abortions. Chronic LAs are “bad actors,” and hemostasis labs put a lot of effort into detecting and confirming that they are present or, more happily, ruling them out.
Of the laboratory results you report, the LA-sensitive partial thromboplastin time (PTT–LA) and the dilute Russell viper venom time (DRVVT) relate to LA, however they are incomplete. Because both are prolonged, lab scientists would say they are presumptively positive for LA, but require confirmation using specially designed, readily available high-phospholipid reagents. It would be a mistake to report the presence of LA without these confirmatory test results, and the patient’s laboratory is probably equipped to perform the tests.
The elevated antithrombin (AT, previously named antithrombin III, AT III) result has no clinical significance. A chronically reduced AT level would associate with many of the disorders that are associated with LA, especially venous thromboembolic disease.
The reduced protein C could be very significant, as low protein C also associates with venous thromboembolic disease. This test, however, is affected by the anticoagulant (“blood thinner”) Coumadin, and must be confirmed by a repeat test performed when the person is in good health and not taking any drugs or dietary supplements. LA, if present, also interferes with the protein C test. We never conclude that someone is protein C deficient without confirmatory testing at a time when we can be assured there are no interfering circumstances.
Sorry for the lengthy answer, but you have hit on one of the more complex issues in laboratory medicine. I hope I’ve been clear, but please feel free to provide additional information or ask follow-up questions, I’m delighted to help.