Dr. Tauseef Ismail Assistant Professor Dept of C Med. KGMC Screening Dr. Tauseef Ismail Assistant Professor Dept of C Med. KGMC

Introduction Screening is the identification of unrecognized disease by application of rapid tests to separate well persons who probably have the disease from those who probably do not have the disease. A screening test is not intended to be diagnostic. Persons with positive results should be referred for diagnosis and treatment.

Characteristics of a Good Screening test Inexpensive Easy to administer Minimal discomfort Reliable (consistent) Valid (distinguishes diseased & non-diseased)

Diagnostic test & Screening test A diagnostic test is used to determine the presence or absence of a disease when a subject shows signs or symptoms of the disease. „ A screening test identifies asymptomatic individuals who may have the disease. „ The diagnostic test is performed after a positive screening test to establish a definitive diagnosis.

Types of screening Mass screening Targeted screening  involves screening a large population (e.g., chest x-rays for TB) Targeted screening Selected groups who are anticipated to have an increased prevalence of the condition for which screening has been instituted (e.g. Measuring the blood cholesterol in relatives of people with familial hyperlipidemia ) OR groups with specific exposures is often used in environmental and occupational health (e.g., battery workers).

Multiple or multiphasic screening involves the use of several screening tests on the same occasion (e.g., an annual health check-up). Case-finding (or opportunistic screening) is a form of screening restricted to patients who consult a health practitioner for some other purpose (the GP may take your blood pressure when you come for your 'flu shot)

Some common Screening tests Pap smear for cervical dysplasia or cervical cancer Fasting blood cholesterol for heart disease Fasting blood sugar for diabetes Blood pressure for hypertension Mammography for breast cancer PSA test (prostate specific antigen) for prostate cancer Fecal occult blood for colon cancer Ocular pressure for Glaucoma PKU test for Phenylketonuria in newborns TSH for hypo & hyperthyroid PSA: Prostate-specific antigen

Quality of Screening Tests Depends on: Validity – Ability of the test to distinguish between who has a disease and who does not A perfect test would be perfectly valid Reliability – Repeatability of a test A perfectly reproducible method of disease ascertainment would produce the same results every time it was used in the same individual.

Validity of Screening Test It has Two components; Sensitivity – the ability of the test to identify correctly those who HAVE the disease; the search for diseased persons Specificity – the ability of the test to identify correctly those who DO NOT HAVE the disease; the search for well persons Sensitivity and specificity quantify a test’s accuracy in the presence of known disease status.

- + a + b - c d True Disease Status Screening Test Results of a = true positive b = false positive c = false negative d = true negative

- + a + b - c d True Disease Status Screening Test Results of Screening Test + - + a b - c d Sensitivity: The probability of testing positive if the disease is truly present Sensitivity = a / (a + c)

- + a + b - c d True Disease Status Screening Test Results of Screening Test + - + a b - c d Specificity: The probability of screening negative if the disease is truly absent Specificity = d / (b + d)

+ - 132 + 983 - 45 63650 Sensitivity: a / (a + c) Sensitivity = Breast Cancer Physical Exam and Mammo- graphy + - + 132 983 - 45 63650 Sensitivity: a / (a + c) Sensitivity = Specificity: d / (b + d) Specificity =

- + 132 983 + 45 63650 - Breast Cancer Physical Exam and Mammo- graphy Sensitivity: a / (a + c) Sensitivity = 132 / (132 + 45) = 74.6% Specificity: d / (b + d) Specificity = 63650 / (983 + 63650) = 98.5%

Sensitivity: Screening by physical exam and mammography will identify 75% of all true breast cancer cases. Specificity: Screening by physical exam and mammography will correctly classify 98.5% of all non-breast cancer patients as being disease free.

Repeatability When there is no satisfactory standard against which to assess the validity of a measurement technique, then examining its repeatability is often helpful.

Reliability (Repeatability) of Tests Can the results of a test be replicated? If a test is valid but NOT reliable, results are meaningless Factors that contribute to variation between test results: Intra-observer variation – variation within individual subjects by the same observer e.g. blood pressure variation in an individual Inter-observer variation – variation between those reading test results Intra-Subject variation- blood pressure, blood sugar, serum cholesterol

Inter-observer Variation Two observers may not always arrive at the same results The extent or magnitude of disagreement is important…if disagreement is large the results are less meaningful or less reliable Variation between observers can be quantified by calculating a percent agreement

Overall percent agreement Analyzing repeatability Overall percent agreement Reading No 1 Reading No 2 Abnormal Suspect Doubtful Normal A B C D Abnormal + Suspect E F G H + I J K L Doubtful + Normal M N O P A + F + K + P × 100 Percent agreement = Total Readings

Inter-Observer Agreement in Grading Severity of Cataract Reliability (reproducibility) Agreement within and between examiners ________________________________________________ Inter-Observer Agreement in Grading Severity of Cataract Examiner 1: Grade Examiner 2 <1 1-<2 2-<3 3-<4 4 <1 10 2 1 0 0 1-<2 1 20 2 0 0 2-<3 0 1 20 1 0 3-<4 0 0 1 10 2 4 0 0 0 2 5 % Agreement = 83.3%

Predictive Value of a test Reflects diagnostic power of a test The probability that a patient with a positive test has in fact the disease in question Positive PV The probability that a patient with a negative test has in fact not got the disease in question Negative PV

Comparing the definitions Sensitivity: The probability of testing positive if the disease is truly present Specificity: The probability of screening negative if the disease is truly absent Positive PV: The probability that a patient with a positive test has in fact the disease in question Negative PV: The probability that a patient with a negative test has in fact not got the disease in question

a + b c + d Disease Status P A + Screening − test a b c d Positive predictive value = probability of the person having the disease when the test is positive = a/a+b Negative predictive value= probability of the person not having the disease when the test is negative = d/c+d

a b c d sensitivity specificity PPV NPV

Criteria for Screening Programs There are WHO guidelines for deciding when screening is appropriate ,drawn up by Wilson and Jungner in 1968; Condition should be an important health problem The natural history should be well understood. There should be a detectable early stage. There should be a suitable test for the early stage. The test should be acceptable to the population to be screened. The cost should be balanced against benefits.

A school nurse routinely tests all new children ages 6-14 for strabismus (having crossed eyes). The prevalence of this condition at this age group is approximately 5 %. The nurse’s examination has a sensitivity and specificity of 90%. She refers those who test positive to the ophthalmologist; whose examination also has sensitivity and specificity of 90%. Q-1. If this nurse examines 2000 children, she is going to refer to the ophthalmologist: Q-2. The predictive value of a positive test by the nurse is:

For Further Reading http://ocw.jhsph.edu/courses/fundepi/pdfs/lecture11.pdf