Regarding PPA and NPA there is an important clarification that...

Regarding PPA and NPA there is an important clarification that shareholders should be aware of, in contrast to sensitivity and specificity.

I will quote it from the FDA:
https://www.fda.gov/RegulatoryInformation/Guidances/ucm071148.htm#4

4.1 Comparisons with the Benchmark
The choice of comparative benchmark and the methods of comparison and reporting are influenced by the existence and/or practical applicability of a reference standard. Depending on the availability of a reference standard, FDA makes the following recommendations regarding the choice of comparative benchmark:

1. If a reference standard is available: use it to estimate sensitivity and specificity
2. If a reference standard is available, but impractical: use it to the extent possible. Calculate estimates of sensitivity and specificity adjusted to correct for any (verification) bias that may have been introduced by not using the reference standard to its fullest extent.
3. If a reference standard is not available or unacceptable for your particular intended use and/or intended use population: consider whether one can be constructed. If so, calculate estimated sensitivity and specificity under the constructed standard.
4. If a reference standard is not available and cannot be constructed: calculate and report measures of agreement (see Appendices).

...

With RAP, I understand we are dealing with point 4.

Namely, agreement.

This is elaborated:

If a reference standard is not available and cannot be constructed

When a new test is evaluated by comparison to a non-reference standard, you cannot directly calculate unbiased estimates of sensitivity and specificity. Therefore, the terms sensitivity and specificity are not appropriate to describe the comparative results. Instead, the same numerical calculations are made, but the estimates are called positive percent agreement and negative percent agreement, rather than sensitivity and specificity. This reflects that the estimates are not of accuracy but of agreement of the new test with the non-reference standard.

In addition, quantities such as positive predictive value, negative predictive value, and the positive and negative likelihood ratios cannot be computed since the subjects’ condition status (as determined by a reference standard) is unknown.

In this situation, FDA recommends you report

• the 2x2 table of results comparing the candidate test with the comparative method
• a description of the comparative method and how it was performed
• the pair of agreement measures along with their confidence intervals.

...

For fellow hotcopperites, there are two things of note.

The first is that the issue is *agreement* between clinical/radiological diagnosis and the RAP cough test is the outcome, not whether or not the person has this or that form of pneumonia. That means both can be wrong. This may not matter that much in FDA approval but definitely does in terms of clinical outcome.

The second issue is confidence intervals. These are recommended by the FDA and so may be expected in the SMARTCOUGH-C trial. This involves some of the best hospitals available so the results will be interesting. If good, expect a force multiplier effect.

If pedestrian, then it's fingers crossed on the other disease options canvassed. On the surface, results on these seem to have higher validity but relatively less utility.

If all else fails, there are definitely other sound signatures I think worthwhile, including the differentiation of pulmonary oedema from asthma, ie cardiac wheeze or volume overload vs bronchospasm.

The underyling technology is not limited to smartphones.