Was there an independent, blind comparison with a reference ('gold') standard of diagnosis?

In considering this question, we need to determine whether all patients in the study underwent both the diagnostic test under evaluation (in our scenario, the serum ferritin) and the reference standard (in our scenario, bone marrow biopsy) to show that they definitely do or do not have the target disorder. We should also ensure that those investigators who are applying and interpreting the reference standard do not know the results from the diagnostic test.

We also need to consider if the reference standard is appropriate. Sometimes a reference standard may not be clear cut, (such as in the diagnosis of delirium) and in this case, we'd need to review the rationale for the choice of reference standard as outlined by the study authors.

All patients in the study we found underwent serum ferritin testing and bone marrow biopsy.

1. Was there an independent, blind comparison with a reference ("gold") standard of diagnosis?
2. Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom we would use it in practice)?
3. Was the reference standard applied regardless of the diagnostic test result?
4. Was the test (or cluster of tests) validated in a second, independent group of patients?