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Centre for Evidence-
Based Medicine

Completed Diagnosis Worksheet for Evidence-Based General Practice

Citation

Holty I, Forster DP. Evaluation of pure tone audiometry and impedance screening in infant schoolchildren. J Epidemiol Community Health 1992; 46: 21-25.

Are the results of this diagnostic study valid?

  1. Was there an independent, blind comparison with a reference ("gold") standard of diagnosis?
    Yes - the audiometry and tympanometry were done by different examiners 5 days apart and without knowledge of the previous result.
  2. Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom it would be used in practice)?
    School-aged children were screened.
  3. Was the reference standard applied regardless of the diagnostic test result?
    Yes - all children were supposed to undergo both tests, and 94.1% did.

Are the valid results of this diagnostic study important?

Your calculations:

Target Disorder (abnormal audiometry) Totals
Present Absent
Diagnostic Test Result
(tympanometry)
Positive
(type B or C)

99

a

92

b

191

a + b

Negative
(type A)

73

c

310

d

383

c + d

Totals

a + c

172

b + d

402

a + b + c + d

574

Sensitivity = a/(a+c)
= 99/172
= 58%

Specificity = d/(b+d)
= 310/402
= 77%

Likelihood Ratio for a positive test result (LR+) = sens/(1-spec)
= 58%/23%
= 2.5

Likelihood Ratio for a negative test result (LR-) = (1-sens)/spec
= 42%/77%
= 0.54

Positive Predictive Value = a/(a+b)
= 99/191
= 52%

Negative Predictive Value = d/(c+d)
= 310/383
= 77%

Pre-test Probability (prevalence) = (a+c)/(a+b+c+d)
= 172/574
= 81%

Pre-test-odds = prevalence/(1-prevalence)
= 30%/70%
= 0.43

Post-test odds = Pre-test odds x Likelihood Ratio

Post-test Probability = Post-test odds/(Post-test odds + 1)

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?
    Yes. Many practices, including ours, have one of these simple cheap instruments.
  2. Can you generate a clinically sensible estimate of your patient's pre-test probability (from practice data, from personal experience, from the report itself, or from clinical speculation)
    Parental concern is a poor predictor of hearing problems (Rosenfeld, Arch Otolaryngol Head Neck Surg 1998 Sep;124(9):989-92). I would adjust the prevalence slightly to a pre-test value of 40%
  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?)
    A positive test would predict about a 63% chance of an abnormal audiogram (and warrant an audiogram); a negative test a 27% chance (and warrant a repeat test in a several weeks).
  4. Would the consequences of the test help your patient?
    Yes - a recent trial (Maw, Lancet 1999 353: 960-3) of delayed versus immediate surgery for OME showed a benefit in language development but that the delayed group also later caught up.

Additional Notes

While the "disease" of interest is otitis media with effusion, I have taken audiometry as the gold standard since it is really the hearing impairment that is important to the child, not the presence of some middle ear fluid.

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