A diagnostic test is any kind of medical test performed to aid in the diagnosis of detection of a suspected disease or condition. This is different from a screening test which is used when not when a disease or condition is suspected, but when people are considered to be at high risk of developing a disease or condition. For example; A physician performing a biopsy on a woman with a lump in one of her breasts is a diagnostic test for suspected breast cancer. A post-menopausal women undergoing a routine mammogram without any breast cancer symptoms is a screening test for breast cancer.

Screening tests are offered to asymptomatic people who may or may not have early disease or disease precursors and test results are used to guide whether or not a diagnostic test should be offered. Diagnostic tests are offered to people who have a specific indication of possible illness (a history, symptom, sign or positive screening test result) to determine whether or not they have the disease in question.

Differences between diagnostic tests and screening tests [1].

 

 Diagnostic test

 Screening test

Result

The cutoff is set towards high specificity, with more weight given to diagnostic precision and accuracy than to the acceptability of  the test to patients

The cutoff is set towards high sensitivity. As a result many of the positive results are false positives. This is acceptable, particularly if the screening test is not harmful or expensive.

Cost

Patients have symptoms that require accurate diagnosis and therefore higher costs are justified.

Since large numbers of people will be screened to identify a very small number of cases, the financial resources needed must be justified carefully.

Result of the test

The test provides a definitive diagnosis (e.g. a definite diagnosis of Meningitis through blood test or lumbar puncture.

The result of the test is an estimate of the level of risk and determines whether a diagnostic test is justified.

Invasiveness

May be invasive (lumbar puncture)

Often non-invasive.

Population offered the test

Those with symptoms or who are under investigation following a positive screening test.

Those at some risk but without symptoms of disease.

 References

1. Sheringham J, Kalim K, Crayford T. Mastering Public Health: A guide to examinations and revalidation. ISBN-13 978-1-85315-781-3