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Centre for Evidence-
Based Medicine

Completed Prognosis Worksheet for Evidence-Based Gastroenterology and Hepatology


Loftus EV, Silverstein MD, Sandborn WJ, et al. Crohn's Disease in Olmstead County Minnesota, 1940-1993: Incidence, Prevalence, and Survival. Gastroenterology 1998;114:1161-1168.

Are the results of this prognosis study valid?

  1. Was a defined, representative sample of patients assembled at a common (usually early) point in the course of their disease?
    Yes - 90% of the population are seen at the Mayo clinic in any 3 year period and record linkage ensures good coverage. Date of onset of symptoms was identified.
  2. Was patient follow-up sufficiently long and complete?
    Yes-minimum of 1 year but records from 1940 onwards were reviewed. Median follow-up=13.3 years (0.1-51.3).
  3. Were objective outcome criteria applied in a "blind" fashion?
    Death certificates were obtained for all patients who died. Causes of death were recorded (table3).
  4. If subgroups with different prognoses are identified, was there adjustment for important prognostic factors?
    No. No difference in survival was observed according to year of diagnosis or sex.
  5. Was there validation in an independent group ("test-set") of patients?

Are the valid results of this prognosis study important?

  1. How likely are the outcomes over time?
    43 patients died (19%). Survival was slightly less than expected:
    20 year survival 73%; expected 86%
    30 year survival 73%; expected 74%
    Only survival for patients diagnosed between 1963 and 1974 was less than expected.
  2. How precise are the prognostic estimates?
    Not stated.

If you want to calculate a Confidence Interval around the measure of Prognosis

Clinical Measure Standard Error (SE) Typical calculation of CI
Proportion (as in the rate of some prognostic event, etc.) where:

The number of patients = n

the proportion of these patients who experience the event = p
where p is proportion and n is number of patients
If p = 43/226 = 0.19(or 19%) and n=226

SE = sqrt((0.19x(1-0.19))/226)
= 0.026 (or 2.6%)

95% CI is 19% +/- 1.96 x 2.6% or 13.9% to 24.1%

Can you apply this valid, important evidence about prognosis in caring for your patient?

  1. Were the study patients similar to your own?
  2. Will this evidence make a clinically important impact on your conclusions about what to offer or tell your patient?

Additional Notes


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