Screening for Disease: A Primer

Sep 13 2011 Published by under [Medicine&Pharma], General Health

Yesterday I reviewed a paper that raised the question of screening urinalysis for adolescents and young adults, showing that asymptomatic isolated hematuria (blood in the urine) indicated almost 20-fold increased risk for development of end-stage kidney failure over twenty years. Does this alone make this a good screening test?

The best discussion I found on screening for disease is a PDF from the National Institute of Occupational Safety and Health (NIOSH) regarding mining-associated lung disorders. A screening test must:

  • Have acceptable sensitivity, specificity, and predictive value
  • Be valid and reliable
  • Be able to identify disease early and lead to treatment that impedes disease progression

So we want a test that is sensitive. This means that it will identify affected individuals at a high rate (essentially 100%), producing very few false negative results (subjects have the condition but test negative). Specificity, or a test that produces few false positives (subjects do not have the condition but test positive), is also nice; however, since we are screening an at risk population who will be subjected to confirmatory tests, it is usually more important to make sure we have no false negatives.

Valid and reliable seem to speak for themselves.

The third point sparks discussion. What if our screening tests do not identify a treatable disorder? Or if treatment can be administered, what if it makes no difference in survival or quality of life? Mass screenings under those conditions make little sense.

Two other points from the NIOSH PDF fall into this same general area:

  • Adequate follow-up, further diagnostic tests, and effective management of the disease must be available, accessible, and acceptable
  • Benefits of the screening program must outweigh the costs

Most patients who screen positive for something must then undergo further confirmatory tests. The purpose of screening is to identify all affected individuals; some false positives can be acceptable in a screening test. Screening for a disorder for which there is no treatment available seems cruel, unless there are other benefits. Knowing one had HIV could prevent spread of the disease, even before we had anti-retroviral therapies.

Finally the benefits of screening must outweigh the costs. All costs, not just the monetary ones, must be considered. The brouhaha over annual mammograms a few years back addressed not just the monetary costs of breast biopsy for false-positive lesions, but the pain, psychological distress, lost wages, and other human costs of those tests.

A screening test ultimately must identify affected individuals who would benefit from early (asymptomatic) detection and treatment of a condition. As in everything in medicine, benefits must outweigh risks and costs. Sometimes the answer is clear (determining maternal Rh blood type early in pregnancy); other times, as with screening UAs, it is not.

 

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