Screening - A Medical Overview PDF

Screening Dr. Hafeez Osman Mahmoud  Outlines of the lecture:  General principles of screening  Characteristics of diseases suitable for screening  Characteristics of a screening test  Appropriate situations for screening  Criteria for instituting screening programme “ Health should...

Screening Dr. Hafeez Osman Mahmoud  Outlines of the lecture:  General principles of screening  Characteristics of diseases suitable for screening  Characteristics of a screening test  Appropriate situations for screening  Criteria for instituting screening programme “ Health should mean a lot more than escape from death or, for that matter , escape from disease”  The iceberg phenomenon What is seen by physician What is hidden GENERAL PRINCIPLES OF SCREENING Detectable by testing outcome Biological Symptoms diagnosed Disabled/ onset begin cured/ died Natural history of many chronic diseases 5  Screening-Definition: The presumptive identification of unrecognized disease or defect by the application of tests, examinations or other procedures that can be applied rapidly in apparently healthy individuals.  To sort out apparently well persons who probably have a disease from those who probably do not. Not intended to be diagnostic. A positive finding or suspicious finding must be referred for diagnosis  Screening finds people that are likely to have a disease or it looks for factors that are precursors to disease.  Screening tests can also be used to identify people that are at greater risk of having a disease  Screening can be considered a form of secondary prevention. Screening differs from periodic examinations in the following respects; 1. Capable for 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 AND DIAGNOSTIC TESTS CONTRASTED SCREENING TEST DIAGNOSTIC TEST ◼Done on ◼Done on those with apparently indication or risk healthy ◼Applied to single ◼Applied to groups patients. All diseases are considered ◼Test results are ◼Diagnosis is not final arbitrary and final but modified in light of new evidence. Diagnosis is the sum of all evidence SCREENING TEST DIAGNOSTIC TEST ◼Based on one ◼Based on evaluation criterion or cut-off of a number of point e.g. (diabetes) symptoms, signs, ◼Less accurate and laboratory ◼Less expensive findings ◼More accurate ◼Not a basis of treatment ◼More expensive ◼The initiative come ◼Used as a basis for from the investigator treatment or agency providing ◼The initiative come care from a patient with complaint Explanation of Terms A. Screening Screening is the test for infection or disease in populations or individuals who are not seeking health care ; ( serological testing for AIDS virus in donors),  B. Case- finding This is the use of clinical/ or laboratory tests to detect disease in individuals seeking care for other reasons for example the use of VDRL (Venereal disease research laboratory test) test to detect syphilis in pregnant women.  C. Diagnostic tests Use of clinical and/or laboratory procedures to confirm or refute the existence of disease in patients with signs and symptoms presumed to be caused by the disease  Classification of Screening Simple or complex ▪ Prostate cancer screening – rectal examination & PSA Individual or mass screening ▪ Among patients who come for check up vs health camps for people Routine or ad-hoc (episodic) ▪ Antenatal screening vs groups with specific exposure Aims of a screening programme: To detect disease predisposition so that individual can be removed from exposure to specific hazards. To detect early stages of disease so that treatment can be given.  Diseases appropriate for screening  Important public health problem  Known natural history Common (high prevalence). Effectively treatable if detected early. Biological Ds detectable Symptoms onset by screening DPCP detectable pre-clinical phase Characteristics of screening tests Validity (Accuracy) of the test ▪ Ability of a measuring instrument to give a true measure Reliability (Precision) ▪ The ability of a measuring instrument to give consistent results on repeated trials. Acceptability  Validity  Should measure the true measurement  Must give a good indication of those who have the disease  The extend to which the test is, capable of correctly diagnosing the presence or absence of the disease concerned.  Validity of screening test increase cost True characteristics in the anxiety, population worry label/stigma Disease No disease TRUE FALSE Positive POSITIVE POSITIVE Screening Test Negative FALSE TRUE NEGATIVE NEGATIVE Disease is serious effective intervention is available Disease can spread Subsequent diagnostic evaluation has minimal cost & risk SENSITIVITY Is the proportion of diseased people who were correctly identified by the test. Frequency with which persons who have the disease test positive a/a+c = those that test positive / all with disease Specificity SPECIFICITYssc Is the proportion of non-diseased people who are correctly identified negative by the test. Is the frequency with which persons who do not have the disease test negative d/b+d = those that test negative / all without disease  Assume a population of 1000 of whom 100 have the disease and 900 do not have the disease. Screening test to identify the 100 people with the Disease True characteristics in the population Disease No disease Results Positive 180 80 100 Screening Test Negative 820 20 800 100 900 Calculate the sensitivity and specificity of the test  ANSWER: True characteristics in the population Disease No disease Results Positive 180 80 100 Screening Test Negative 820 20 800 100 900 Sensitivity = 80/100 Specificity = 800/900 = 80% = 89 %  Validity… Public health importance: how good is the test at identifying people with disease & without disease. Clinical setting: physician would like to know ……if the test is positive in this patient, what is the probability that this patient has the disease?  Predictive value of a test:  Positive predictive value o What proportion of the patients test positive actually have the disease? o A high positive PV - indicated good cost benefit.  Negative predictive value o What proportion of the patients test negative do not have the disease? Predictive value Predictive value A/aa Number who test of a positive = a / a + b = positive with test (PV+) disease/ Number with positive result Predictive value = d / c + d = Number who test of a negative negative without test (PV-) disease/ Number with negative result  Predictive value Test result Disease No Total disease Positive 80 100 180 Negative 20 800 820 Total 100 900 100 Calculate a) Positive Predictive Value b) Negative Predictive Value  Predictive value Test result Disease No Total disease Positive 80 100 180 Negative 20 800 820 Total 100 900 100 Positive Predictive Value = 80/180 = 44% Negative Predictive Value = 800/820 = 98% Predictive value (PV) Is influenced by  Disease prevalence prevalence PV +  Specificity of the screening test specificity PV + Acceptability Final attribute of a good screening test. Acceptable : Simple, safe Easily administered Procedures that are acceptable Should not have unpleasant or potentially hazardous implications  Reliability  Ability of the test to give the same results when repeated examinations are performed on the same persons under the same conditions.  Reliability (precision, reproducibility) affected by: o Biological variation o Variation due to the test method / measurement o Intra-observer variation o Inter-observer variation  Cont…  Increasing reliability of the test: oStandardization of procedures oIntensive training oPeriodic checks oUse of two or more observers making independent observations Appropriate situations for screening 1. Social Important for the individual & community. Available diagnostic, follow-up & intervention to all. Favorable cost-benefit ratio. High public acceptance. 2. Scientific Adequately understood natural history of condition Sound case definition & a policy regarding who to treat as patients High prevalence of disease or condition  3. Ethical  Benefit vs. harm  Informed consent  Efficacy of screening proved in a proper trial Criteria for instituting screening programs  Disease: oSerious, high prevalence oNatural history understood  Diagnostic tests: oValid, oReliable, oSimple & cheap oSafe & acceptable (low side effects, convenient, painless)  Diagnosis & treatment: oFacilities adequate oEffective, acceptable & safe treatment available.  Other considerations: oSocial oethical Uses of screening 1. Case detection 2. Control of disease 3. Research purposes 4. Educational opportunities Conclusion  Fundamental issues need to be looked into when considering disease prevention by screening.  A number of criteria must be considered carefully before a decision is made to implement a screening programme.  Ethico-legal consideration.  Cannot assumed that a screening programme will benefit the population. Read it again