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Technically speaking, you are correct - you need to know the sensitivity to calculate the PPV. However, you can derive an upper bound on the PPV by taking the sensitivity to be 100%. The upper bound calculated in this way (20÷1019) would be more appropriately rounded up to 2.0%.
However, the point of the letter is that doctors massively overestimate the PPV. Most doctors answered 95%, so it doesn't matter whether the real PPV is 2.0%, 1.9% or less - we have a real problem affecting our medical decision making here.
Kind regards,
Dr. Rupert Millard
Competing interests:
No competing interests
11 June 2017
Rupert A Millard
GP
Heatherlands Medical Centre
396 New Hey Road, Woodchurch, Wirral, CH49 9DA, UK
I do not believe it is possible to calculate the PPV of the test (= TP/TP+FP) on the basis of the data provided in the letter (prevalence and FP rate). The sensitivity of the test should have been added.
Sensitivity missing, but not that important
Dr. Piero Baglioni,
Technically speaking, you are correct - you need to know the sensitivity to calculate the PPV. However, you can derive an upper bound on the PPV by taking the sensitivity to be 100%. The upper bound calculated in this way (20÷1019) would be more appropriately rounded up to 2.0%.
However, the point of the letter is that doctors massively overestimate the PPV. Most doctors answered 95%, so it doesn't matter whether the real PPV is 2.0%, 1.9% or less - we have a real problem affecting our medical decision making here.
Kind regards,
Dr. Rupert Millard
Competing interests: No competing interests