How to do Tourniquet Test for Dengue
- A tourniquet test determines capillary fragility.
- The test is part of the WHO algorithm for diagnosis of Dengue fever.
- A blood pressure cuff is applied and inflated.
- The test is positive if there are more than 10 to 20 petechiae per square inch.
In a tourniquet test for dengue, the blood capillaries are examined for their ability to withstand increased pressure. This gives a good indication for assessing the blood vessels and whether it will lead to Dengue Shock Syndrome (DSS) or Dengue Hemorrhagic Fever (DHF).
This test involves checking what are known as petechiae, i.e. small red spots under the surface of the skin. These petechiae are formed when blood leaks from capillaries into the skin. Pressing the petechiae does not have any effect on its appearance.
Performing the Dengue Tourniquet Test
The old version of the test was to simply apply a tourniquet (any kind) for 5 minutes at the upper arm until red dots appear distal to it. A variety of factors had to be considered: the type of tourniquet, the pressure applied, and the results to determine if it is positive. So a standard was developed by medical schools of long ago:
- First measure the blood pressure. You need the systolic BP and the Diastolic BP to determine the standard pressure for the Tourniquet test which is the mean pressure (DBP + SBP divided by 2).
- If you take the blood pressure slowly, you will find tiny red or salmon-colored dots distal to the cuff. This is the best place to mark your test area. Also this area is usually the anterior or volar aspect of the forearm distal to the elbow crease. If you don't find any dots, mark the mentioned area anyway with a black ink marker using either a circle or a square with an area of 2.5 cm2.
- Reinflate the blood pressure cuff at the computed mean pressure.
- Hold it inflated for five minutes. This is very painful. Reassure the patient that this is normally felt during the test. Of note, some residents have taught medical students like me that this should be 20 minutes. This is wrong and unethical.
- Count the total number of petechiae visible on the test area. It is positive if 20 or more petechiae are seen in the area. Some of the petechiae do not resolve. Dengue patients that are in the critical or afebrile phase may develop ecchymosis. This can also be interpreted as positive.
This test is not considered to be completely reliable for confirming dengue. Factors that can influence the results are premenstrual and postmenstrual period in women and sun damaged skin in people. This is because all such people are sure to have relatively fragile capillaries. Dengue Tourniquet Test does have its use in diagnosis of dengue. It is useful in monitoring a patient’s condition when he is suffering from Dengue Shock Syndrome (DSS) or Dengue Hemorrhagic Fever (DHF).
The World Health Organization (WHO) has defined this test as one of the necessary requirements for dengue fever diagnosis. In case of Dengue Haemorrhagic Fever (DHF), the test generally gives a sure positive result with 20 or more petechiae per square inch. Since the test does not have high specificity i.e. ability to confirm the infection, the interfering conditions must be taken into consideration.
Premenstrual women, postmenstrual women who are not taking hormonal supplements, women having skin damaged by sun must not take this test as they have high capillary fragility due to altogether different reasons. Since dengue infection is typical of a tropical and sub-tropical country, the tourniquet test is used to evaluate the possibility of dengue infection immediately in such countries. There are 36 million symptomatic cases of dengue worldwide with 2.1 million severe cases reported in the year 2010. With a tourniquet test, people suspected of having contracted the infection can be immediately hospitalised for treatment and further diagnosis.
Read more articles on Dengue Diagnosis and Prognosis
Source: Onlymyhealth editorial team Apr 04, 2011
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