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Diagnosis Critical Appraisal Worksheet

Download this worksheet [Word document; 29kb]

Citation

Are the results of this diagnostic study 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 it would be used in practice)?
Was the reference standard applied regardless of the diagnostic test result?

Are the valid results of this diagnostic study important?

Sample calculations

show/hide sample calculations

		Target Disorder (iron deficiency anaemia)		Totals
		Present	Absent	Totals
Diagnostic Test Result (serum ferritin)	Test Positive (< 65 mmol/l)	731 a	270 b	1001 a + b
Diagnostic Test Result (serum ferritin)	Negative (≥ 65 mmol/l)	78 c	1500 d	1578 c + d
Totals		a + c 809	b + d 1770	a + b + c + d 2579

Sensitivity = a/(a+c)
Sensitivity = 731/809
Sensitivity = 90%

Specificity = d/(b+d)
Specificity = 1500/1770
Specificity = 85%

Likelihood Ratio for a positive test result = LR+
Likelihood Ratio for a positive test result = sens/(1-spec)
Likelihood Ratio for a positive test result = 90%/15%
Likelihood Ratio for a positive test result = 6

Likelihood Ratio for a negative test result = LR-
Likelihood Ratio for a negative test result = (1-sens)/spec
Likelihood Ratio for a negative test result = 10%/85%
Likelihood Ratio for a negative test result = 0.12

Positive Predictive Value = a/(a+b)
Positive Predictive Value = 731/1001
Positive Predictive Value = 73%

Negative Predictive Value = d/(c+d)
Negative Predictive Value = 1500/1578
Negative Predictive Value = 95%

Pre-test Probability (prevalence) = (a+c)/(a+b+c+d)
Pre-test Probability (prevalence) = 809/2579
Pre-test Probability (prevalence) = 31%

Pre-test-odds = prevalence/(1-prevalence)
Pre-test-odds = 31%/69%
Pre-test-odds = 0.45

Post-test odds = Pre-test odds x Likelihood Ratio for a positive result
Post-test odds = 0.45x6
Post-test odds = 2.7

Post-test Probability = Post-test odds/(Post-test odds + 1)
Post-test Probability = 2.7/3.7
Post-test Probability = 73%

Your calculations

		Target Disorder (iron deficiency anaemia)		Totals
		Present	Absent	Totals
Diagnostic Test Result (serum ferritin)	Test Positive	a	b	a + b
Diagnostic Test Result (serum ferritin)	Test Negative	c	d	c + d
Totals		a + c	b + d	a + b + c + d

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

Is the diagnostic test available, affordable, accurate, and precise in your setting?
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)? (Are the study patients similar to your own?; Is it unlikely that the disease possibilities or probabilities have changed since the evidence was gathered?)
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?)
Would the consequences of the test help your patient?

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