SCREENING TESTS, CRITERIA & INDICATORS. SCREENING PROGRAMMES IN COMMUNITY HEALTH ��DR ROSELIN. V�ASSOCIATE PROFESSOR

Definition Screening

“The search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals”

Points considered before screening programme

• Applied to people most likely to benefit from it.
• Selection – Age, Sex, Medical History, Occupation, Family History etc.
• Choice of test – Based on compromise.
• Not done in isolation – Integrated into health services.
• Risks, Benefits – Explained to participants.

Aims and Objectives

Individuals or population not seeking health care

• HIV – Blood donors.
• Premarital syphilis screening.
• Neonatal screening.

Case finding - Use of clinical or laboratory test

• VDRL in pregnant women.

Diagnostic test – patients with signs & symptoms

• VDRL in patients with lesions on genitals.

Uses

Case detection

• “Prescriptive screening” – Presumptive identification of unrecognised disease not arising from patients request like, Neonatal screening.

Control of disease

• “Prospective screening” for benefits of others – e.g. immigrants.

Research Purposes

• Chronic diseases – cancers, hypertension, initial screening for prevalence, next screening for incidence
• No followup therapy available.

Educational opportunities

• Public awareness
• Educating health professionals.

Conserving physicians time.

Types

Mass screening

• Whole population.
• Backed by treatment.

High risk (Selective screening)

• Productive, effective life,
• Diabetes, Hypertension – Accumulate in population subgroups.
• Diabetes in 40 and above.
• Prostrate screening in 60 and above.

• Screening for risk factors – Preventive measures applied before actual disease. like,
• Serum cholesterol - coronary artery disease

Multiphasic screening

• Two or more screening tests are applied.
• Questionnaires, clinical examination, investigation, measurement.

Opportunistic screening

• Restricted to patients consulting doctor.

Advantages

• Improved prognosis – Elimination, postponment of death from disease.
• Non fatal conditions – Improved quality of life style.
• Resources for treatment and followup – Less consumption.
• Correct negative test – Reassures those without disease.

Disadvantages

• Prolonged period of morbidity – e.g. cancer.
• Diagnosis of a pseudo disease – over treatment of healthy screening (False positive).
• False negatives – not diagnosing underlying disease.
• Test itself risky – mammography for breast cancer.
• Expensive technology – Increased financial cost for health services.

Model for early detection programmes

Screening and incidious profile of disease

Apparently healthy (Screening tests)

• Apparently normal (Periodic re-screening)

• Apparently abnormal
• Normal periodic- screening
• Intermediate- surveillance
• Abnormal - treatment

Difference from Periodic health examination

• Capable of wider application.
• Relatively inexpensive.
• Requires little physician time.

Criteria

Disease to be screened

• Important health problem – high prevalence.
• Recognize latent or early asymptomatic stage.
• Natural history – latent phase – declared disease.
• Effective tests to detect disease.
• Facilities – confirm diagnosis.
• Effective treatment.
• Agreed on policy – whom to treat as diseases like borderline diabetes – cut off levels.
• Evidence – early detection, treatment reduces morbidity, mortality.
• Expected benefits greater than risk and cost of screening.

Screening tests

Acceptability

• Co-operation – vaginal examination, rectal examination.

Repeatability

• Reliability, precision or reproducibility.
• Consistency.
• Depends on 3 major factors
• Observer variation.
• Biological variation.
• Errors relating to technical methods.

Observer variation

• Intra observer.
• Inter observer.
• Common in – X-rays, ECG, BP readings, histopathological specimens etc.

Minimisation

• Standardising procedures for measurements classification - Calibration.
• Training observers.
• Two or more observers for independent assessment.
• Repetition of observations and taking average.

Biological variation

• Physiological variables – BP, Blood sugar, serum cholesterol etc.

Reasons

• Change in parameter observed – frequent in clinical presentation like cervical smears, MI.
• Way patient perceives symptoms and answer – questionnaire, past events.
• Regression to mean – ulcerative colitis, rheumatoid arthritis.

Errors - technical method

• Defective instruments.
• Errors in calibration.
• Faulty reagents.
• Test itself may be innappropriate or unreliable.

Validity (Accuracy)

• Def – ability of a test to distinguish those who have disease from those who do not – glycosuria, GTT.
• Two components – Specificity and sensitivity (expressed in percentages)

Screening result by diagnsosis

Screening indicators

• Sensitivity = TP / (TP+FN) X 100
• Specificity = TN / (TN+FP) X 100
• Predictive value
• Positive test = TP / (TP+FP) X 100
• Negative test = TN / (FN+TN) X 100
• Percentages
• False negative = FN / (TP+FN) X 100
• Positive = FP / (FP+TN) X 100

Sensitivity

• Ability of a test to identify correctly all those who have disease – true positives.

Specificity

• Ability of a test to identify correctly those not having disease – True negative.

Diagnosis of brain tumors by EEG

• Sensitivity = 36 / 40 X 100 = 90%
• Specificity = 306 / 360 X 100 = 85%

Diagnosis of brain tumors by Computer assisted tomography

• Sensitivity = 39 / 40 X 100 = 97.5%
• Specificity = 342 / 360 X 100 = 95.0%

Others

• Yield – Amount of previous unrecognised disease diagnosed as a result of screening effort.
• Depends on
• Sensitivity, specificity.
• Prevalence of disease.
• Participation of individuals.
• High risk populations selected for screening – 40 yrs above adults for diabetes.

Predictive accuracy

• Diagnostic power of test.
• Depends on
• Sensitivity
• Specificity
• Disease prevalence
• Prevalence directly proportional to predictive accuracy.

Predictive value of positive gram stained cervical smear test (with constant sensitivity of 50% and specificity of 90%) at three levels of prevalence

False negatives

• Harmful in serious diseases, time for rescreening.
• Sensitivity indirectly proportional to false negatives.

False positives

• Subjected to further diagnostic tests.
• Anxiety, inconvenience.
• Specificity is inversely proportional to false positives

Problem of borderline

Cut-off point - factors

Disease prevalence

• When high screening, set at a lower level.
• Increases sensitivity.

Disease

• Very lethal like cancer – greater sensitivity is desired.
• Proportion of false positives tolerable but not false negatives.
• False positives ruled out by further diagnostic test.
• Diabetes Mellitus – high specificity is desired, false positives decreased.
• Useful index – predictive value of positive test.

References

• Oxford textbook of public health by Walter W. Holland, Rager Deteb and George Knox, 4^th Edition.
• Textbook of Community Medicine by Sundar Lal, Adarsh, Pankaj 1^st Ed.
• Textbook of preventive and social medicine by K. Park, 19^th Edition, 2007.