because they have asked about THE TEST, and sensitivity/specificity are the properties of the test. whereas PPV & NPV are dependant upon the population being tested, it's not the intrinsic property of the test.
Another way to think of this: She has a strong family history, so we are thinking she probably does indeed have this mutation (probably a True Positive). Our fear, would be we do the wrong test and aberrantly tell her that she is in the clear (False Negative). Having a high False Negative would be deleterious to this patient, and plugging this into a 2x2 table gives a low sensitivity (TP/ TP +FN).
I picked the wrong one because Fecal Occult Blood Testing (FOBT) is high-sensitivity. My thought process is even if it is positive it won't tell use anything specific will need to scope anyways.
https://nccrt.org/wp-content/uploads/FOBTCliniciansReferenceFinal.pdf
Even i got it wrong, then searched google and got this from Harrison (FOBT has low sensitivity and better methods are now available for screening) : https://harrisons.unboundmedicine.com/harrisons/view/Harrisons-Manual-of-Medicine/623689/all/COLORECTAL_CANCER - PREVENTION : Early detection of colon carcinoma may be facilitated by routine screening of stool for occult blood (Hemoccult II, ColonCare, Hemosure); however, sensitivity only โผ50% for carcinoma; specificity for tumor or polyp โผ25โ40%. Newer tests (e.g., Cologard) incorporating detection of blood and mutated genes are more sensitive and specific. False positives for occult blood: ingestion of red meat, iron, aspirin; upper GI bleeding. False negatives: vitamin C ingestion, intermittent bleeding. Genetic testing is unaffected by these factors.
So, to rule OUT a disease (Sen-Out/snout), screening test should have high sensitivity. Vice versa, to rule IN a disease (Spec-In/specin), confirmatory test should have high specificity.
I still don't understand how we rule out NPV and PPV from the answers since we're specifically talking about the 'at-risk' population group only and these values are used for that itself.
submitted by โhayayah(1212)
Sensitivity tests are used for screening. Specificity tests are used for confirmation after positive screenings.
Sensitivity tests are used for seeing how many people truly have the disease. Specificity tests are for those who do not have the disease.
A highly sensitive test, when negative, rules OUT disease. A highly specific test, when positive, rules IN disease. So, a test with with low sensitivity cannot rule out a disease. A test with low specificity can't rule in disease.
The doctor and patient want to screen for colon cancer and rule it out. The doctor would want a test with high sensitivity to be able to do that. He knows that testing her stool for blood will not rule out the possibility of colon CA.