Was the reference standard applied regardless of the diagnostic test result?
We need to check to see that even if a patient's serum ferritin was normal, the study investigators performed the reference standard. Sometimes if the reference standard is invasive, it may be considered unethical to perform it on patients with a negative test result. For example, if a patient with chest pain is suspected to be at low risk of a pulmonary embolism and has a negative V/Q scan, an investigator (who is performing a study looking at the accuracy of the V/Q scan in diagnosing pulmonary embolism) may not want to subject the patient to pulmonary angiography which is not without morbidity and mortality. Indeed, this was what the investigators did in the PIOPED study - if patients were considered to be at a low risk of a pulmonary embolism and had a negative V/Q scan, rather than undergoing a pulmonary angiogram, they were followed up clinically for several months, without receiving antithrombotic therapy to see if an event occurred.
In the ferritin study, all patients received both the diagnostic test and the reference standard.
- Was there an independent, blind comparison with a reference ("gold") standard of diagnosis?
- Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom we would use it in practice)?
- Was the reference standard applied regardless of the diagnostic test result?
- Was the test (or cluster of tests) validated in a second, independent group of patients?