Iceberg phenomenon of disease
Epidemiologists and others who study disease find that the pattern of disease in hospitals is quite different from that in a community. That is, a far larger proportion of disease (e.g., diabetes, hypertension) is hidden from view in the community than is evident to physicians or to the general public. The analogy of an iceberg , only the tip of which is seen, is widely used to described disease in the community.
Concept of Screening :
"The search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals". Screening differs from periodic health examinations in the following aspects:
1) capable of wide application
2) relatively inexpensive, and
3) require little physician-time In fact the physician is not required to administer the test, but only to interpret it.
SCREENING TEST :
* It is done on apparently healthy individuals
* Applied to groups
* Test results are arbitrary and final
* Based on one criterion or cut-off point
* Less accurate
* Less expensive
* Not a basis for treatment
* The initiative comes from the investigator or agency providing care.
"Lead time"- It is the length of time between the detection of disease and its usual clinical presentation and diagnosis.
"A" is the usual outcome of the disease,
"B" is the outcome to be expected when the disease is detected at the earliest possible moment. The benefits of the programme are therefore B-A. It is important to consider the outcomes when evaluating the advantages. Any detection program's expenses and complexity must be weighed against the advantages that result from it.
AIMS & OBJECTIVES OF SCREENING :
* The basic purpose of screening is to sort out from a large group of apparently healthy persons those likely to have the disease or at increased risk of the disease .
* They also brings, "apparently abnormal" under medical supervision and treatment.
Uses of Screening
Four main uses have been described:
A) Case detection
B) Control of disease
C) Research purposes
D) Educational opportunities
Screening test- The test must satisfy the criteria of acceptability, repeatability and validity, besides others such as yield, simplicity, safety, rapidity, ease of administration and cost.
Tests most likely to fulfil one condition may however, be least likely to fulfil another - for example, tests with greater accuracy may be more expensive and time consuming. The choice of the test must therefore often be based on compromise.
Examples of Screening Tests:
Pap smear, mammogram, clinical breast exam, blood pressure determination, cholesterol level, eye examination/vision test, and urine analysis.
What are sensitivity and specificity?
Sensitivity and specificity are measures of a test's ability to correctly classify a person as having a disease or not. A highly sensitive test means that there are few false negative results, and thus fewer cases of disease are missed. It may not be feasible to use a test with low specificity for screening, since many people without the disease will screen positive, and potentially receive unnecessary diagnostic procedures.
The specificity of a test is its ability to designate an individual who does not have a disease as negative.
EVALUATION OF A SCREENING TEST
- Sensitivity = True Positive Fraction = P(Screen Positive | Disease) = a/(a+c)
- Specificity = True Negative Fraction = P(Screen Negative | Disease Free) = d/(b+d)
One might also consider the:
- False Positive Fraction = P(Screen Positive | Disease Free) = b/(b+d)
- False Negative Fraction = P(Screen Negative | Disease) = c/(a+c) .
Reference : Park Textbook of PSM.