The EuroQol questionnaire has 5 items, each with 3 levels of response, which gives 243 (3^5) possible combinations. The EQ-5D index converts this 1-dimensional matrix into a single number anchored to go from 0 (death) to 1 (good health). However, some patterns give a negative index.
Questionnaire: EuroQol
Matrix: (mobility) (self-care) (usual activities) (pain or discomfort) (anxiety or depression)
The precise pattern is mathematically transformed to an index based on the population being studied.
Some of the matrices associated with a negative index are 32331, 33232, 33323 and 33333.
Since 0 is defined as death, a negative index is referred to as "worse than death" (WTD).
When evaluating a patient for orthopedic surgery, a negative index due to the defective body part (and not due to comorbidities) often is associated with maximal benefit from the surgery.
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