Veterinary Epidemiology and Zoonosis

MEASURES OF VALIDITY OF A DIAGNOSTIC TEST

Diagnostic test and screening test

• Diagnostic tests play major role in medical decision-making.
• The results of a diagnostic test used to decide to initiate treatment, to determine the level of treatment and to determine the frequency of disease in the world.
• A distinction must be made between a diagnostic test and a screening test.
• Diagnostic tests are used to distinguish between animals that have the disease in question and those that have other diseases on the differential list.
• Diagnostic testing begins with diseased individuals.
• Screening tests are used for presumptive identification for unrecognized disease / detect in apparently healthy population.
• Screening test begins with presumably healthy individuals.

Test accuracy

• It is the proportion of all tests both positive and negative that are correct, also termed validity.
• It is used to express overall performance of a diagnostic test.
• The test accuracy answers the question ‘what is the likelihood the test result is correct’.
• The standard of validity (gold standard) is the proof of the diagnosis and is a quality control device that provides the basis for determining the value of diagnostic test, treatment strategies and prognosis.

Properties of diagnostic tests

• Sensitivity
• Test sensitivity is defined as the likelihood of a positive test result in animals known to have the disease (aT+/D+). It is also defined as the ‘true positive’.
• Specificity
• Test specificity is defined as the likelihood of a negative test result in animals known to be free from the disease (aT-/D-). It is also defined as the ‘true negative’.
• False positive
• It is the likelihood of a positive test result in animals to be free from the disease (aT+/D-). It equals to 1- specificity.
• False negative
• It is the likelihood of a negative test result in animals known to have the disease (aT-/D+). It equals to 1- sensitivity.
• Predictive values
• The probability that a test result reflects the true disease status.
• Positive predictive value
• It is the probability of the disease in an animal with positive test result (pD+/T+).
• Negative predictive value
• It is the probability of that animal doesn’t have the disease when the test result is negative (pD-/T-).
• Sensitivity and specificity have absolute properties of a test and do not change for any given cutoff value.
• Whereas predictive values are relative, varying with prevalence of a disease in the population. It is evaluated by 2x2 table.

Test	DISEASE		Total
	Present	Absent
Positive	a (True)	b (False)	a+b
Negative	c (False)	d (True)	c+d
Total	a+c	b+d	a+b+c+d