Table 1

Types of biases introduced through diagnostic testing

	Type of bias		Recognising bias	Effect on accuracy
	Partial verification workup or referral bias		Only patients tested with gold standard are included; patients with positive index test are more likely to get gold standard.	Falsely increases sensitivity by lowering rate of false negatives
Part 1: Suboptimal patient selection	Spectrum bias through case–control design		Inclusion of ‘sickest of the sick’ or ‘wellest of the well’	Falsely increases sensitivity and specificity
	Spectrum bias through dropping indeterminate subjects		Ask ‘did they describe their method for handling indeterminates?’	Falsely increases sensitivity if excluded indeterminates have mild disease. Falsely increases specificity if excluded indeterminates are not diseased.
	Spectrum bias through convenience sampling		Look for screening modality in methods section.	Falsely elevates sensitivity and specificity when sampling excludes difficult, indeterminate or ambiguous patients.
Part 2: Diagnostic and verification test	Interpretation	Indeterminate	When indeterminate results are considered dichotomously as positive or negative.	It can overestimate or underestimate the accuracy of the test depending on how indeterminates are included.
	Interpretation	Review	Occurs when the person interpreting the diagnostic test has access to the gold standard test.	Usually falsely increases the sensitivity and specificity of the index test.
	Verification	Incorporation	Occurs when the criteria for a gold standard include the results of the diagnostic test	Falsely increases the sensitivity and specificity of the index test.
	Verification	Double gold standard (differential verification)	Occurs when gold standard test is invasive or expensive, and is only performed when index test result is positive.	Falsely increases the sensitivity and specificity of the index test.

Bold text denotes the biases discussed in this manuscript, ‘Recognising Bias in Studies of Diagnostic Tests Part 1: Suboptimal Patient Selection’ 1. Greyed out text denotes the biases discussed in ’Recognising Bias in Studies of Diagnostic Tests Part 2: Interpreting and Verifying the Index Test'.