Dr swe swe latt screening for slideshare

ScreeningScreeningDR. SWE SWE LATT M.B.,B.S, M.Med.Sc (Public Health)Advanced Diploma in Business Management (ABE(UK))LECTURERDEPARTMENT OF COMMUNITY MEDICINE Kulliyyah of Medicine, International Islamic University Malaysia

Learning outcomes Describe the aims and objectives of the screeningDescribe the differences between Screening & Diagnostic testsList the uses of screening Explain the types of screening, criteria for screeningDiscuss the Validity of the screening test Calculate and interpret the evaluation of the screening test

SCREENING

SCREENING

Definition

“ the search for unrecognized disease or

defect by means of rapidly applied

tests, examinations or other procedures

in apparently healthy individuals”

(K. Park 20th Edition)

Definition“ the process of using tests on a large scale to

identify the presence of disease in apparently

healthy people” (R. Bonita 2nd Edition)

“Tests done in individuals with no symptom or

sign of an illness are referred to as screening

tests” (J H Abramson 5th Edition)

• Disease/ defect

• Apparently healthy

Definition of screening

• Whom

• Test/ examination

• Unrecognized d/s-

• Rapidly applied tests

Aims & Objectives

•To sort out from a large group of apparently

healthy persons those likely to have the

disease or at increased risk of disease

•To bring those who are apparently

abnormal under medical supervision and

treatment

Explanation of terms Screening, Case finding & Diagnostic testsScreening Case-Finding Diagnostic test

testing for infection or disease in population or in individuals who are not seeking health care

E.g. Serological testing for

HIV in blood donors,

Premarital screening for

syphilis

use of clinical and laboratory tests to detect disease in individuals seeking health care for other reasons

Eg. the use of VDRL test to detect syphilis in pregnant women, Pul TB in chest symptomatics, hypertension, cervical ca, breast ca, DM

Use of clinical and laboratory procedures to confirm the existence of disease or true abnormality in patients with signs and symptoms presumed to be caused by the disease. Eg. VDRL testing of patients with lesions suggestive of secondary syphilis.

Differences: Screening & Diagnostic testsScreening test Diagnostic test

1 Done on apparently healthy Done on those with indicationor sick

2 Applied to group Applied to single patient

3 Results are arbitrary & final Not final

4 Base on one criterion or cut off point

Base on evaluation of S/S & laboratory findings

5 Less accurate More accurate6 Less expansive More expansive7 Not a basis for treatment Basic for treatment8 From investigator or agency From patient with complaints

Uses of screening

1. Case detection Prescriptive screening, people are screened for their own benefit(Ca Breast, Diabetes, HT)

2. Control of disease Prospective screening, people are screened for the benefit of others ( TB, HIV, STI)

3. Research Purpose To know the natural history of a disease

4. Educational opportunities

Public awareness

1. Case detection Prescriptive screening, people

are screened for their own benefit

(Ca Breast, Diabetes, HT)2. Control of disease

Prospective screening, people

are screened for the benefit of

others ( TB, HIV, STI)3. Research Purpose

To know the natural history of a

disease4. Educational opportunities Public awareness

CONCEPT OF LEAD TIME IN SCREENING

Lead time is defined as the interval between the time disease can be first diagnosed by screening and that when it is usually diagnosed in patients presenting with symptoms .

In the figure, 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 benefit of the program is B – A (lead time).

A

B

TYPES OF SCREENING

1) Mass screening

2) High risk or selective screening

(Targeted screening)

3) Multiple or multiphasic screening

4) Case- finding or Opportunistic screening

1. Mass screening

The screening of a whole population or a subgroup (e.g. all adults)

Example: a. Visual defects in school

children b. Mammography in women

aged 40 years or lessc. Newborn screening

program in Japan

2. High risk/selective/ targeted screening

The screening of selected high-risk groups in the population.

Example: a. Screening fetus for Down’s

syndrome in a mother who already has

a baby with Down’s syndrome

a. screening for Ca cervix in Low SES women

b. Screening of HIV in risk groups

3. Multiple or Multiphasic screening

Various diagnostic procedures are employed during the screening program.

Example:- DM- FBS, Glucose

tolerance test (GTT)(Health questionnaire,

Clinical examination, Measurements, Investigations (chemical & haematological tests on blood & urine specimens)

4. Case-finding or Opportunistic screening

Aimed at patients who consult a health practitioner for some other purpose.

Example: • RHD in children

Gold standard – Infection ( Culture), Ca (Biopsy), Drug testing ( RCT), Cause of death ( Autopsy)

Disease Serious, significant burden of diseaseHigh prevalence of preclinical stageNatural History understoodAppropriate test available for early detection of diseaseFacilities for diagnosis of diseaseEffective treatment available for diseaseLong period between first signs and overt disease

Diagnostic test Sensitive and specific Simple and cheap Safe and acceptableReliable ( repeatability)

Diagnosis and treatment

Facilities are adequateEffective, acceptable, and safe treatment available

Criteria for instituting a screening programme

SCREENING TESTCheap

Easy to apply

Acceptable to the public

Reliable / repeatability (consistent results)

Valid (accurate)

• Validity (accuracy) - what extent the test accurately measures which it purports to measure.

(Ability of a test to separate or distinguish those who have the disease from those who do not)

It has 2 main components: Sensitivity Specificity

Validity of Screening est

Screening test result

DiagnosisDisease Not disease

Total

PositiveNegative

a bc d

a + bc + d

Total a + c b + d a+ b +c+d a= Number of true positivesb= Number of false positivesc= Number of false negativesd= Number of true negatives

Disease present Disease absentTest positive

True positive(TP)(a)

False positive(FP)(b)

Total positive(a+b)

Test negative

False negative(FN)

(c)

True negative(TN)(d)

Total negative

(c+d)

Sensitivity The ability of a test to give positive results in a group of persons with the diseaseProbability of a positive test in people with disease (true positive)

Sensitivity =TP x 100=a/ (a+c) x 100 TP+ FN

90% sensitivity – 90 % of the diseases people screened by the test will give a “true positive” result and the remaining 10% “ a false negative” result .

Disease present Disease absent

Test positive



Total positive(a+b)

Test negative

False negative(FN)

(c)


Total negative(c+d)

SpecificityThe ability of a test to give negative results in a group of persons without the diseaseProbability of a negative test in people without the disease (true negative)

Specificity =TN x 100 = d/ (b+d) x 100 TN+ FP

90% specificity – 90 % of the non-diseased persons will give “true negative” result, 10% of non- diseased people screened by the test will be wrongly classified as “diseased” when they are not.


Test positive



Total positive(a+b)

Test negative

False negative(FN)

(c)


Total negative(c+d)

Efficiency of a test

Represented by the percentage of all true

positive and true negative results.

Efficiency = TP+ TN / Grand Total x 100

= a+ d / a+ b+ c+ d x 100


Test positive



Total positive(a+b)

Test negative

False negative(FN)

(c)


Total negative(c+d)

Predictive value of a positive test- The probability of the person having the disease when

the test is positive- PPV= a/ a+ b x 100Predictive value of a negative test - The probability of the person not having the disease

when the test is negative- NPV = d/ c+d x 100


Test positive



Total positive(a+b)

Test negative

False negative(FN)

(c)


Total negative(c+d)

Percentage of false negative- The patients who actually have the disease

are told that they do not have the disease. - c/ a+c x100Percentage of false positive - The patients who do not have the disease

are told that they have. b/ b+d x 100


DiagnosisDisease

Not disease

Total

PositiveNegative

a c

b d

a + bc + d

Total a + c b + d a+ b +c+d

YIELD- The amount of previously unrecognized disease

that is diagnosed as a result of the screening effort.

- Yield = TP+ FP/ TP+FP+FN+TN (a+b/ a+b+c+d)

It depends upon- Sensitivity of the screening test- Prevalence of the disease-The participation of individual in the detection

program Hence, yield of a screening is high in high-risk

screening.


DiagnosisDisease

Not disease

Total

PositiveNegative

a c

b d

a + bc + d


Evaluation of a screening test

(a) Sensitivity = a / (a+c ) x 100 (b) Specificity = d / (b+d) x 100(c) Predictive value of a positive test (PPV) = a / (a+b) x 100(d) Predictive value of a negative test (NPV) = d / (c+d) x 100



Total

PositiveNegative

a bc d

a + bc + d


(e) Percentage of false negative = c /(a+c )x 100(f) Percentage of false positive = b /(b+d )x 100(g) Efficiency of the test = a+ d / (a+b+c+d)



Total

PositiveNegative

a b c d

a + bc + d


Some screening tests

Pregnancy AnaemiaHTVDRL testCardiovascular disease

Neural tube defectsDown’s syndromeHIV

Middle-aged men and women HTCaDM Serum cholesterol Obesity

InfancyCongenital heart diseaseSpina bifidaHearing defectsVisual defectsUndescended testis

ElderlyNutritional disordersCancerTB Chronic bronchitisGlaucoma Cataract

Common screening tests Sensitivity & Specificity

Fasting Blood sugar for Diabetes(5 mmol /l)

Sensitivity – 85- 89%Specificity – 70 -79%

PSA test for Prostate Cancer(4ng/ml)

Sensitivity -20-32%Specificity – 94-97%

PAP smear for cervical cancer Sensitivity – 29-56%Specificity – 94-100%

Mammography for breast cancer

Sensitivity -75-95%Specificity -83-98%

Fecal occult blood for colon cancer

Blood pressure for HT

PROBLEMPROBLEMThe result of biopsy histological findings and

mammogram are used to screen Breast cancer, among 500 women aged 40 and above.

Out of the 500 screened, 150 were suggestive of Ca. breast with Mammogram, while only 120 biopsy result were confirmed for presence of Tumour. In 30 of the instances, Mammogram results were not suggestive of Cancer, while histological biopsy revealed the presence of it.

1) Construct 2 x 2 table

2) Sensitivity

3) Specificity

4) Predictive value of positive test

5) Predictive value of negative test

6) Percentage of false positive

7) Percentage of false negative

Screening test

Results (Memography)

Diagnosis (Biopsy)

TotalDisease(+)

Not Disease

(-)

Positive (+) 120 (a) 30 (b) 150(a+ b)

Negative (-) 30 (c) 320 (d) 350 (c+ d)

Total 150 (a+ c) 350 (b+ d) 500 (a+b+c+ d)

Sensitivity = a/ a+c =120/ 150 x 100 = 80%

Specificity = d/ b+d = 320/ 350 x 100 = 91.4%

- Predictive value of positive test (% of positives with the disease)

= a/ a+b =120/150 x100 = 80%

-Predictive value of negative test (% of negatives without the disease)

= d/ c+d =320/ 350 x 100 = 91.4%

-Percentage of false positive = b/ b+d

=30/350x 100 = 8.5% - Percentage of false negative test = c/ a+c = 30/150x 100 = 20%Efficiency of the test = (TP+ TN)/Grand total = (a+ d)/( a+b+c+d) x 100 = 440/ 500 x 100 = 88%

QUIZ BONUS

‘The active search for unrecognized disease or defect in

apparently healthy people using rapidly applied tests or

procedures’ is

A. Case finding

B. Active surveillance

C.Screening

D.Monitoring

E. All of the above

False

False

True

False

False

QUIZ BONUS

The time interval between diagnosis by early

detection and diagnosis by other means is:

A.Serial interval

B.Lead time

C.Time lag

D.Latent period

False

True

False

False

QUIZ BONUS‘Multiphasic screening’ means:

A. Application of two or more screening tests in combination

at one time

B. Application of two or more screening tests in combination

at different time

C.Application of two or more screening tests in combination

at different geographical areas

D.Application of separate screening tests for a

single disease

True

False

False

False

QUIZ BONUS

In addition to the acceptability and cost

effectiveness, a good screening test should be:

A. Repeatable

B. Not repeatable

C. High specific though

may not be sensitive

A. None of the above

True

False

False

False

QUIZ BONUS

Sensitivity of a screening test refers to:

A. True negative

B. True positive

C. False negative

D. False positive

False

True

False

False

The property of a test to identify the proportion of truly ill persons in a population who are identified as ill by a screening test

A.SensitivityB.SpecificityC.Positive predictive valueD.Negative predictive value Answer : A

• The probability of a persons having the disease when the test is positiveA. SensitivityB. SpecificityC. Positive predictive valueD. Negative predictive value

Answer: C

• The extent to which a test is measuring what it is intended to measureA. ReliabilityB. ValidityC. SensitivityD. Specificity

Answer: B

References

(K. Park Textbook of PSM 23rd Edition 2015)

(R. Bonita 2nd Edition)- J H Abramson and Z H Abramson Survey methods in Community Medicine 5 th Edition)Basic Epidemiology WHO (R. Bonita, R. Beaglehole)

Health & Medicine

Dr swe swe latt screening for slideshare