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Centre for Evidence-
Based Medicine

Completed Diagnostic Worksheet for EBM in Developing Countries

Citation

Collantes E, Velmonte MA. The Validity of the Typhidot Test in Diagnosing Typhoid Fever Among Filipinos. Phil J of Med - Infectious Dis 1997; 26:61-63.

Are the results of this diagnostic study valid?

  1. Was there an independent, blind Comparison with a reference ("gold") standard of diagnosis?
    Yes, blood culture was done in all study patients. Although the authors stated that blood culture was the supposed gold standard, it seems an inappropriate reference or gold standard. The diagnosis at present is still predominantly dependent on the clinical course of the patients. Patients whose clinical course is highly suggestive of typhoid fever are given antibiotics for typhoid fever even while still awaiting the results of blood culture (which takes at least 3 days to be obtained) or even with negative culture results. In the article, there was confusion as to the real reference standard that was considered by the authors. They stated that blood culture is the gold standard but during the analysis, it seems that they considered the clinical diagnosis as the gold standard. However, the authors did not clarify whether clinical course was used for the diagnosis, per se and not diagnosis on admission.
  2. Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom it would be used in practice)?
    Yes. The study was done in a tertiary general hospital in an urban area which admits various cases of Infectious and Tropical Diseases. These include the differential diagnoses in the scenario which are prevalent in the setting where the study was conducted.
  3. Was the reference standard applied regardless of the diagnostic test result?
    Yes. Although the authors said that this was a retrospective study, patients included were those with results of both the typhidot and blood culture. In addition prior to the study period, the consultants informed the residents of the service that both blood culture and typhidot tests should be requested for all patients suspected of typhoid fever.

Are the valid results of this diagnostic study important?

** An analysis completely different from what the authors did is presented in the following tables because of problems with the gold standard.

IgM Clin Dx of Ty (+) Dx of Ty fever (-) Likelihood Ratio
IgM (+) 96 0 (1*) 49.71
IgM (-) 16 57 0.14
Totals 112 58
* substituted 1 for 0 for computation purposes
IgG Blood Culture (+) Blood Culture (-) Likelihood Ratio
IgG (+) 87 36 1.23
IgG (-) 25 21 0.22
Totals 112 57

Can you apply this valid, important evidence about a diagnostic test in caring for your patient?

  1. Is the diagnostic test available, affordable, accurate, and precise in your setting?
    Variable in every setting. The price varies in the type of center where it is done. In a government hospital, indigent patients may avail of the laboratory free of charge. Even in private patients (those with capacity to pay) variations exist as some might be charged lower than US$ 12.50 but others could be charged at more than US $ 25.00. In addition, there is no data with regards to reliability of results from any center.
  2. Can you generate a clinically sensible estimate of your patient's pre-test probability (from patient data, from personal experience, from the report itself, or from clinical speculation)
    Approximately 50% based on history/PE.
  3. Will the resulting post-test probabilities affect your management and help your patient? (Could it move you across a test-treatment threshold? Would your patient be a willing partner in carrying it out?)
    Using the (+) result of the IgM test brings the pre-test probability of 50% to a post-test probability to 98%, while using the IgG (+) result in a separate analysis (not serially) does not change the pre-test probability markedly ( a little over 50%). A negative IgM will result to a post-test probability of approximately 7.5%. The test results in IgM led to marked changes in the pre-test probability and definitely moved across a test-treatment threshold. However since this test is obtained only at one point in the course of the disease, treatment for typhoid fever might still be given if the clinical course turned out to be still highly suggestive of typhoid fever. It might be interesting to determine if a repeat typhidot test after several days would still yield similar results.
  4. Would the consequences of the test help your patient?
    In the scenario mentioned, the results of the IgM test and not the IgG tests led helpful movements across treatment threshold.