The snare test is a quick exam that must be done in all cases of suspected dengue fever, as it allows the identification of blood vessel fragility, common in dengue virus infection.
This test can also be known as a tourniquet test, Rumpel-Leede test or simply capillary fragility test, and is part of the World Health Organization recommendations for the diagnosis of dengue, although this test is not always positive in people with dengue. It is for this reason that, after the positive result, a blood test must be done to confirm the presence of the virus.
As it identifies the risk of bleeding, the noose test need not be used when there are already signs of bleeding, such as bleeding gums and nose or the presence of urine blood. In addition, the snare test may present false results in situations such as the use of aspirin, corticosteroids, pre- or post-menopausal phase, or when there is sunburn, for example.
What is the exam for
The snare test is mainly known to aid in the diagnosis of dengue, however, as it tests the fragility of the vessels, it can also be used when you are suspicious of other diseases that can cause bleeding, such as:
- Scarlet fever; Thrombocytopenia; Hemophilia; Liver diseases; Anemia.
Since the loop test can be positive in several situations, after knowing the result it is always recommended to do other diagnostic tests, starting with blood tests, for example.
How the test is done
To do the loop test you should draw a square on the forearm with an area of 2.5 x 2.5 cm and then follow these steps:
- Assess the person's blood pressure with the sphygmomanometer; Inflate the sphygmomanometer cuff again to the average value between maximum and minimum pressure. In order to know the average value, it is necessary to add the Maximum Blood Pressure with the Minimum Blood Pressure and then divide by 2. For example, if the blood pressure value is 120x80, the cuff should be inflated to 100 mmHg; Wait 5 minutes with the cuff inflated at the same pressure; Deflate and remove the cuff after 5 minutes; Let the blood circulate for at least 2 minutes.
Finally, one must evaluate the amount of reddish spots, called petechiae, within the square on the skin to find out what the test results are.
How to understand the result
The loop test result is considered positive when more than 20 red dots appear within the square marked on the skin. However, a result with 5 to 19 dots may already indicate suspicion of dengue, and other tests should be done to help confirm whether or not the infection exists.
It is important to remember that the test can be false negative even in people who have the disease, so if there is suspicion through the symptoms, the doctor should request other evaluations to confirm. In addition, it can be positive in other diseases that cause capillary fragility and risk of bleeding, such as other infections, immunity diseases, genetic diseases or even, use of medicines such as aspirin, corticosteroids and anticoagulants, for example.
Thus, it can be observed that this test is not very specific and should be done only to assist in the diagnosis of dengue. Learn more about the tests available to diagnose dengue.
