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Centre for Evidence-
Based Medicine

Completed Prognosis Worksheet for Evidence-Based General Practice

Citation

Kamihira O, et al Long-term stone recurrence rate after extracorporeal shock wave lithotripsy. J Urol 1996 Oct;156(4):1267-71

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?
    903 consecutive patients without residual fragments 3 months after lithotripsy were enrolled.
  2. Was patient follow-up sufficiently long and complete?
    Mean followup was 25 months; with maximum followup beyond 5 years.
  3. Were objective outcome criteria applied in a "blind" fashion?
    Plain abdominal films and/or excretory urograms were evaluated every 6 months.
  4. If subgroups with different prognoses are identified, was there adjustment for important prognostic factors?
    Recurrence rates were higher with multiple stones and pyuria.
  5. Was there validation in an independent group ("test-set") of patients?
    No.

Are the valid results of this prognosis study important?

  1. How likely are the outcomes over time?
    At 25 months mean followup, 183 of 903 patients (20.3%) had had a recurrence. Kaplan-Meier curves show recurrence of:
    6.7% (1 year), 28.0 (3 years) and 41.8% (5 years) respectively.
  2. How precise are the prognostic estimates?
    20% recurrence at 25 months; 95% CI 17%-23%.

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
sqrt((px(1-p))/n)
where p is proportion and n is number of patients
If p = 24/60 = 0.4 (or 40%) and n=60

SE = sqrt((0.4x(1-0.4))/60)
= 0.063

95% CI is 40% +/- 1.96 x 6.3% or 27.6% to 52.4%
n from your evidence: ________
p from your evidence: ________
sqrt((px(1-p))/n)
where p is proportion and n is number of patients
Your calculation:

SE: ____________
95% CI:

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

  1. Were the study patients similar to your own?
    My patient is likely to be at lower risk (single small stone, no pyuria, first episode).
  2. Will this evidence make a clinically important impact on your conclusions about what to offer or tell your patient?
    Yes - the risk is lower than expected, hence we have not considered prophylaxis for this episode.

Additional Notes

Several other studies were found but none looked at a cohort of ALL first presentations of renal colic. The filters in these studies will generally tend to overestimate the recurrence rate.

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