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SCREENING

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Why screen? Who wants to screen? Doctors Labs Hospitals Drug companies Public

Who doesn’t ?

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Screening for Obesity Do you feel obese? Have you ever felt obese? In the last month how often have you felt obese?1-3 times, 4-10 times, more than 10 times How long does each episode of obesity last?Less than 1 hour, 1-2 hours, more than 2 hours Have you discussed your obesity with someone? Who?Spouse, friend, doctor, pastor (religious figure) Have you ever shopped at oversize or “big and tall”

clothing stores?Once, two to 5 times, more than 5 times

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Disease Screening

Criteria for useful screening

•Disease should be:•Common•Serious•Treatable•Slow to become symptomatic•Treatment in asymptomatic phase has to be superior to treatment after symptoms develop

•Test should be:•Inexpensive•Acceptable t patients and physicians•Have high predictive values in the population to be screened

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Disease Screening

How to screen: which tests?

The effectiveness of a screening test depends on its predictive values. These are determined by sensitivity, specificity, and prevalence in the screened population.

The lower the prevalence, the higher the sensitivity and specificity have to be to justify use of the test.

If prevalence is too low, even with high sensitivity and specificity, the great majority of positives will not have the disease, and screening may be indefensible due to the high cost of following up all the healthy positives.

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Disease Screening

Simplicity, cost, and safety: –The dictum "do no harm" is primary with screening, since it will be applied to many people for whom there is no evidence for the disease.

Thus, the screening test must be simple, cheap and safe.

Acceptability to patients and physicians:–If patients won't submit to it, or physicians won't recommend it or accept its results, forget it.

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Disease Screening

Risks of screening Cost/risk of test. False positives

– psychological damage– risk of further testing– cost of further testing

False negatives – may delay diagnosis Labeling

– psychological– medical

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Disease Screening

Labeling effects:– Statement of a diagnosis has an effect on the

patient, distinct from any effects of the disease or its treatment. "Sick" vs. "Healthy" self-images and roles.

False positives, particularly due to multiple testing:– Beware screening by purely statistical criteria of

"abnormality."

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Disease Screening: Biases in evaluating screening programs

Lead-time biasIf time to outcome (e.g., death) is measured from point of diagnosis, early diagnosis will increase the time to outcome (e.g., the length of survival) by the interval between diagnosis by screening, and when diagnosis would have occurred by ordinary means.

This can make early diagnosis appear to increase survival time, even when it has had no effect, or even a damaging effect.

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Disease Screening: Biases in evaluating screening programs

LEAD TIME BIAS

Disease Symptom StandardOnset Onset Diagnosis Death

Diagnosis byScreening

Observed Survival afterStandard Diagnosis

Observed Survival afterScreening Diagnosis

Lead Time

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Disease Screening: Biases in evaluating screening programs

Length-time bias

The proportion of slowly progressing disease picked up by screening will be greater than the proportion picked up by standard clinical practice, since rapidly progressing disease will tend to become symptomatic more quickly.

Therefore, patients diagnosed by screening will, as a group, progress more slowly than those diagnosed by conventional means, even if early treatment has no impact.

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Disease Screening: Biases in evaluating screening programs

LENGTH TIME BIAS

Onset

Diagnosis

Death

Screening Intervention

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Disease Screening: Biases in evaluating screening programs

Compliance bias

Volunteers for screening are generally more health conscious/concerned than the general population, apt to assume greater responsibility for their own care, hence, more likely to comply with therapy.

So groups detected by screening may do better than others, not because early treatment matters, but because they comply with treatment more than others. This is not due to early detection by screening, but to the same reason that made them volunteer for screening in the first place. It is a "volunteer" bias.

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Interpreting Medical Tests and Other Evidence

Some take-home points

Single and combination diagnostic tests can be wrong.

Even correct tests just add information to what you already have, modifying the probabilities that a disease is present or absent. They don’t replace other clinical information from an hx and px.

The degree to which a test result warrants clinical action depends, strongly, on the circumstances of the test -- in particular, on the best assessment from all your contextual and clinical knowledge, of the chance of disease before the test is made. Identical test results may have entirely different implications for different patients, or in different environments.

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Interpreting Medical Tests and Other Evidence

Some take-home points

Highly accurate tests or test sequences (Se and Sp both high) can be clinically useless for some patients.

In particular:– when disease is unlikely to be present before the test, only a

test with very high specificity can establish its presence (rule it in). Otherwise, most positive results will be errors. Such results can harm patients.

– When disease is fairly likely to be present before a test, only a test with very high sensitivity can establish its absence (rule it out). Otherwise, most negative results will be errors. Such results can harm patients.

When a test can’t serve the desired purpose, it should not be ordered.