Screening for Diseases

Screening refers to application of a test to people who are asymptomatic for the purpose of classifying them with respect to the likelihood of having a particular disease.

It is not primarily a diagnostic test

Before screening is conducted, a decision must be made to know if it is worthwhile based on scientific, ethical and even financial justifications

In disease screening, 2 considerations are very important:

• Disease to be screened
• Test to be performed

Mass screening

• ALL eligible population is subjected to a particular screening test
• Expensive and time-consuming
• Gives poor yield

Selective or High-risk screening

• The test or intervention is given to people who have propensity of developing a particular problem
• Gives greater yield within limited time and resources

Multi-phasic screening

• It is done in stages
• Individuals or a large populations are subjected to a variety of tests to rule out the presence of several diseases, at one time.

The criteria for screening are based on 2 considerations:

• The DISEASE to be screened, and
• The TEST to be applied:

Disease Criteria

The Disease to be screened should fulfil the following criteria before it is considered suitable for screening:

• The disease to be screened for must be of public health importance
• There should be a recognizable early asymptomatic stage or Latent stage of the disease - Natural history of the disease must be known
• There must be a test, with high level of accuracy, for detecting the disease prior to onset of signs and symptoms
• Facility for diagnosis and treatment must be available
• Effective treatment must be available
• Evidence that early detection and treatment reduces morbidity and mortality - Expected benefits of early detection must exceed the Risks and Cost
• There should be an Agreed Policy on whom to treat as patient

When the above criteria are satisfied, then only, it would be appropriate to consider a suitable screening test.

Test Criteria

Features of a good screening test

• Acceptability of test (must be acceptable to the population)
• High Repeatability or reliability or precision or reproducibility
• The repeatability of the test depends upon there major factors, namely
• Observer variation
• Intra-observer variation (within observer)
• Inter-observer variation (between observers)
• Biological (subject) variation
• Error relating to Technical methods
• There must be Validity OR Accuracy (i.e. highly sensitive and highly specific) – refers to what extent a test accurately measures what it intends to measure. Accuracy has 2 components: (1) sensitivity and (2) specificity
• Sensitivity is the ability of a test to identify correctly those that have a particular disease i.e. those with Positive screening test.
• Specificity is the ability of a test to identify correctly those who do not have a particular disease.

1. Case detection
2. Research purposes
3. Control of disease
4. Educational opportunity: to train people on risk factor of disease

Sensitivity (True-Positives)

Defined as the ability of the test to be +ve, if the disease is truly present i.e. the ability of a test to identify correctly those who have the disease i.e. true-positives

Specificity (True-Negatives)

The ability of test to be –ve if the disease is truly absent; i.e. ability of a test to identify correctly those who do not have the disease. i.e. “true-negatives”

Predictive value

Reflects the diagnostic power of the test

Positive predictive values (PPV) or ‘predictive value of a positive test’

• Indicates the probability that a patient with a positive test result has, in fact, the disease in question.
• The more prevalent a disease is in a given population, the more accurate will be the predictive value of a positive screening test; as prevalence decreased the predictive value of a positive test falls

Negative predictive value (NPV)

• Indicates the probability that a patient with a negative test result does not, in fact, have the disease in question.

False positives and False Negatives

Whereas the epidemiologist thinks in terms of sensitivity and specificity, the clinician thinks in terms of false negatives and false positives.

False-Negatives:

• The term "false-negative" means that patients who actually have the disease are told that they do not have the disease.
• A screening test which is very sensitive has few "false negatives". The lower the sensitivity, the larger will be the number of false negatives.

False-Positives:

• Means that patients who do not have the disease (i.e. normal healthy persons) are told that they have the disease.
• A screening test with a high specificity will have few false positives. False positives not only burden the diagnostic facilities, but they also bring discredit to screening programmes.

Yield

It is the amount of previously unrecognized disease that is diagnosed as a result of screening efforts.

It depends on many factors:

• Sensitivity
• Specificity
• Prevalence of the disease --- that determines +ve and –ve predictive value
• The participation of the individuals in the detection program.

Pregnancy

• Anaemia
• Hypertension Toxaemia
• Rh status
• Diabetes
• Neural tube defects

Middle-aged men and women

• HTN
• Cancer
• DM
• Serum Cholesterol
• Obesity

Infancy

• Spinal bifida
• Congenital heart disease
• SCD
• Hearing defect

Elderly

• Nutritional disorder
• Cancer
• TB
• Chronic bronchitis
• Glaucoma