VIVO Infectious Disease

Infectious Diseases of Man and Animals

Validity of Diagnostic Tests

There are several standard parameters used to describe the validity of quantitative laboratory tests such as serologic assays and pathogen isolation. In order to determine these factors, some independent and reliable test must be applied to the same samples or samples from the same individual.

Useful descriptions of the following parameters and associated online calculators can be found at MSD References, which also describes additional assessments such as receiver-operator curves and likelihood ratios.

Sensitivity and Specificity

Sensitivity and specificity are the most commonly used indicators of the reliability of a diagnostic test and are typically published for commercial test kits.

Sensitivity	Specificity
Proportion of patients with disease that test POS	Proportion of patients w/o disease, that test NEG
If diseased, how often is the test positive?	If not diseased, how often is the test negative?
Negative test: high certainty the subject does not have disease	Positive test: high certainty the subject does have disease
High sensitivity helps rule out disease	High specificity helps rule in disease
Calculation:	Calculation:
True Positives    True Positives + False Negatives	True Negatives    True Negatives + False Positives

Positive and Negative Predictive Values

PPV and NPP are additional important parameters for test performance and validity. Note that unlike sensitivity and specificity, predictive values changes with the prevalence of the disease in the population being tested. For example, the PPV is higher in populations with more cases.

Note also that PPV and NPV can be calculated based on prevalence or more simply using true and false positive and negatives.

Positive Predictive Value	Negative Predictive Value
The probability that a patient has the disease given a positive test result.	The probability that a patient does not have the disease given a negative test.
True Positives / (True Positives + False Positives)	True Negatives / (True Negatives + False Negatives)
Calculation:	Calculation:
sensitivity x prevalence    (sensitivity x prevalence) + ((1 – specificity) x (1 – prevalence))	specificity x (1 – prevalence)    (specificity x (1 – prevalence)) + ((1 – sensitivity) x prevalence))

Send comments to Richard Bowen: rabowendvm@gmail.com