Are the results of this study valid?
Returning to our clinical scenario from the question formulation tutorial:
You admit a 75 year old woman with community-acquired pneumonia. She responds nicely to appropriate antibiotics but her hemoglobin remains at 100 g/l with an MCV of 80. Her peripheral blood smear shows hypochromia, she is otherwise well and is on no incriminating medications. You contact her family physician and find out that her Hgb was 105 g/l 6 months ago. She has never been investigated for anaemia. A ferritin has been ordered and comes back at 40 mmol/l. You admit to yourself that you're unsure how to interpret a ferritin result and don't know how precise and accurate it is.
In the tutorial on clinical questions we formulated the following question: In an elderly woman with hypochromic, microcytic anaemia, can a low ferritin diagnose iron deficiency anaemia?
Our search of the literature to answer this question retrieved an article from the Am J of Medicine (1990;88:205-9).
How do we critically appraise this diagnosis paper? We'll start off by considering validity first and the following list outlines the questions that we need to consider when deciding if a diagnosis paper is valid.
- 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?