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Centre for Evidence-
Based Medicine

Skin Cancer: General Practitioner examinations are not sufficient for identifying

Purchasing Bottom Line

GP examinations for skin cancer have high specificity but low sensitivity, so are good for reliably referring patients with positive results, but should not be the sole means of identification.

Citation

Whited JD, Hall RP, Simel DL, Horner RD. Primary care clinicians' performance for detecting actinic keratoses and skin cancer. Arch Intern Med 1997;157:985-990.

Purchasing/Policy Question

In patients with skin lesions, what is the accuracy of the general practitioner's diagnosis of skin cancer? Should Primary Care Groups employ alternative means to routine examination for detecting skin cancer?

Search Terms

Best Evidence search using "skin cancer" and "diagnosis." For MEDLINE search, use skin cancer AND diagnosis AND exp sensitivity and sensitivity.

The Study

Blinded comparison of GP versus dermatologist examinations for skin cancer, single actinic keratoses and multiple actinic keratoses in 190 men recruited from dermatology and general medicine outpatient clinics. The reference standard was dermatologist opinion. Each patient was independently examined by a GP and a dermatologist. Interobserver agreement of patients and of lesions was also measured.

The Evidence

Target Disorder
(skin cancer, according to dermatologist exam)
Present Absent
Diagnostic Test Result
(GP exam)
Positive

35

a

15

b

50

a + b

Negative

26

c

114

d

140

c + d

Totals

a + c

61

b + d

129

a + b + c + d

190

Sensitivity = a/(a+c)
= 35/61
= 57%

Specificity = d/(b+d)
= 114/129
= 88%

Likelihood Ratio for a positive test result = LR+
= sens/(1-spec)
= 57%/12%
= 4.9 (rounding off at the final stage)

Likelihood Ratio for a negative test result = LR-
= (1-sens)/spec
= 43%/88%
= 0.48 (rounding off at the final stage)

Positive Predictive Value = a/(a+b)
= 35/50
= 70%

Negative Predictive Value = d/(c+d)
= 114/140
= 81%

Pre-test Probability (prevalence) = (a+c)/(a+b+c+d)
= 61/190
= 32%

Pre-test-odds = prevalence/(1-prevalence)
= 32%/68%
= 0.47

Post-test odds = Pre-test odds x Likelihood Ratio for a positive result
= 0.47x4.9
= 2.3

Post-test Probability = Post-test odds/(Post-test odds + 1)
= 2.3/3.3
= 0.70

Comments

The paper compares GP with dermatologist exams for diagnosing skin cancer. It does not compare GPs or dermatologists reliably against the gold standard of biopsy.

Appraised By

Anna Donald and Sam Vincent March 1999.

Expiry Date

March 2001