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- Question 4: Has work-up bias been avoided? (Greenhalgh, 1997)

This is easy to check. It simply means, "Did everyone who got the new diagnostic test also get the gold standard, and vice versa?" There is clearly a potential bias in studies where the gold standard test is performed only on people who have already tested positive for the test being validated.

- Question 5: Has expectation bias been avoided? (Greenhalgh, 1997)

Expectation bias occurs when pathologists and others who interpret diagnostic specimens are subconsciously influenced by the knowledge of the particular features of the case—for example, the presence of chest pain when interpreting an electrocardiogram. In the context of validating diagnostic tests against a gold standard, all such assessments should be "blind."

- Question 6: Was the test shown to be reproducible? (Greenhalgh, 1997)

If the same observer performs the same test on two occasions on a subject whose characteristics have not changed, they will get different results in a proportion of cases. Similarly, it is important to confirm that reproducibility between different observers is at an acceptable level.

- Question 7: What are the features of the test as derived from this validation study? (Greenhalgh, 1997)

All the above standards could have been met, but the test might still be worthless because the sensitivity, specificity, and other crucial features of the test are too low—that is, the test is not valid. What counts as acceptable depends on the condition being screened for. Few of us would quibble about a test for colour blindness that was 95% sensitive and 80% specific, but nobody ever died of colour blindness. The Guthrie heel-prick screening test for congenital hypothyroidism, performed on all babies in Britain soon after birth, is over 99% sensitive but has a positive predictive value of only 6% (it picks up almost all babies with the condition at the expense of a high false positive rate), and rightly so. It is more important to pick up every baby with this treatable condition who would otherwise develop severe mental handicap than to save hundreds the minor stress of a repeat blood test.

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