What is the mechanism of action (MOA) of the tourniquet test?

Mechanism of Action of the Tourniquet Test

The tourniquet test works by creating controlled capillary fragility assessment through application of pressure that reveals petechiae formation, primarily used in diagnosing dengue infection by demonstrating increased vascular permeability and fragility.

How the Tourniquet Test Works

The tourniquet test is performed using the following procedure:

• A blood pressure cuff is inflated on the upper arm to a point midway between systolic and diastolic blood pressure
• The pressure is maintained for 5 minutes
• After releasing the pressure, the number of petechiae (small red or purple spots) that appear below the pressure site are counted
• The test is considered positive when there are 20 or more petechiae per square inch (6.25 cm²) on the forearm 1

Physiological Mechanism

The tourniquet test functions through several key physiological processes:

1. Increased capillary pressure: The applied pressure causes venous stasis while allowing arterial inflow, increasing hydrostatic pressure in capillaries
2. Vascular fragility assessment: In conditions like dengue, where vascular fragility is increased, this pressure causes small capillaries to rupture
3. Petechiae formation: The ruptured capillaries lead to petechial hemorrhages visible on the skin surface

Diagnostic Applications

The primary application of the tourniquet test is in diagnosing dengue infection:

• It is one of the clinical parameters recommended by the World Health Organization for diagnosing dengue hemorrhagic fever 2
• However, its diagnostic utility is limited - studies show sensitivity of only 41.6% for dengue infection, though specificity is higher at 94.4% 2
• A meta-analysis of 16 studies with 28,739 participants found pooled sensitivity of 58% (95% CI, 43%-71%) and specificity of 71% (95% CI, 60%-80%) 3

Variations in Technique

• Standard method: Uses a sphygmomanometer cuff, which produces more petechiae than alternative methods 2
• Simple elastic tourniquet: Less sensitive but more practical in resource-limited settings 2
• Modified threshold: Using a threshold of 10 petechiae (rather than WHO's recommended 20) increases sensitivity to 45% with 85% specificity 2

Clinical Relevance and Limitations

Despite being included in diagnostic algorithms, the tourniquet test has significant limitations:

• Low sensitivity means a negative test does not exclude dengue infection 2, 4
• The test adds diagnostic value in only about 5% of cases where other evidence of bleeding is absent 2
• Interobserver agreement is generally good (90.2%, Kappa = 0.76) 4
• The test performs slightly better for dengue hemorrhagic fever (sensitivity 62%) than for non-severe dengue (sensitivity 55%) 3

Practical Considerations

When performing the tourniquet test:

• Ensure proper pressure application - midway between systolic and diastolic blood pressure
• Maintain pressure for exactly 5 minutes
• Count petechiae in a standardized area (typically 2.5 cm²)
• Interpret results cautiously, recognizing the test's limited sensitivity
• A positive test should prompt close observation or early hospital referral, but a negative test does not exclude dengue 2

The tourniquet test remains valuable in resource-limited settings where laboratory confirmation may be delayed or unavailable, but clinicians should be aware of its limitations and not rely on it as the sole diagnostic criterion for dengue infection.