At a PHC NCD clinic, the medical officer lowers the fasting plasma glucose level used to call a screening result positive from 126 mg/dL to 110 mg/dL. What is the expected effect?

More cases caught, more false positives too

No change in either type of error

Fewer false positives, more cases missed

More cases caught, fewer false positives

A 52-year-old man attends the medical OPD of a CHC for a sprained ankle. The doctor also checks his blood pressure, finds it raised and arranges follow-up. Which type of screening is this?

Opportunistic screening (case finding)

Mass screening of the population

Multiphasic screening at a camp

Screening: sensitivity, specificity, PPV, NPV

30 NEET-PG practice questions on Screening: sensitivity, specificity, PPV, NPV, part of Community Medicine. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A new rapid test for syphilis is evaluated in 1,000 adults at an STI clinic, of whom 100 have syphilis by the reference test. The rapid test is positive in 90 of the people with syphilis and in 90 of those without it. What is the positive predictive value of the rapid test?
1. A98.8%
2. B50%
3. C10%
4. D90%
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Answer: B · 50%
The 2x2 table is: true positives 90, false negatives 10, false positives 90 and true negatives 810. PPV = true positives / all positives = 90/(90 + 90) = 50%, so half of the people who test positive actually have syphilis. Sensitivity (90/100) and specificity (810/900) are both 90%, and NPV is 810/820 = 98.8%. With the same test, PPV falls further as prevalence falls, which is why positive screening tests in low-prevalence groups need confirmation.
- A: 98.8% is the negative predictive value.
- C: 10% is the prevalence in this group.
- D: 90% is the sensitivity (and the specificity), not the PPV.
Question 2 · Medium
At a PHC NCD clinic, the medical officer lowers the fasting plasma glucose level used to call a screening result positive from 126 mg/dL to 110 mg/dL. What is the expected effect?
1. ANo change in either type of error
2. BMore cases caught, more false positives too
3. CFewer false positives, more cases missed
4. DMore cases caught, fewer false positives
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Answer: B · More cases caught, more false positives too
For a continuous measurement, the cut-off decides the trade-off between sensitivity and specificity. Lowering it calls more people positive: more of the diabetics are caught (sensitivity rises, fewer false negatives), but more healthy people are also labelled positive (specificity falls, more false positives). Raising the cut-off does the opposite. Both cannot rise together by moving the cut-off alone; only a better test shifts the whole ROC curve upwards.
- A: Sensitivity and specificity change whenever the cut-off moves.
- C: This is the effect of raising the cut-off.
- D: Moving the cut-off trades one against the other; both cannot rise.
Question 3 · Medium
A 52-year-old man attends the medical OPD of a CHC for a sprained ankle. The doctor also checks his blood pressure, finds it raised and arranges follow-up. Which type of screening is this?
1. AHigh-risk selective screening
2. BMass screening of the population
3. CMultiphasic screening at a camp
4. DOpportunistic screening (case finding)
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Answer: D · Opportunistic screening (case finding)
Opportunistic screening, or case finding, is the use of a clinical contact made for an unrelated reason to look for another disease, here hypertension during a visit for an ankle sprain. It is cheap and often well accepted, because the patient is already with the doctor. Mass screening covers a whole population regardless of risk, and selective screening is limited to groups at higher risk. Multiphasic screening applies several tests at once, often in a camp.
- A: Nothing marks him out as belonging to a selected high-risk group.
- B: Mass screening invites the whole population, not one attending patient.
- C: One test during a routine visit is not multiphasic screening.
Question 4 · Medium
After a first round of population-based screening for hypertension in a district, the programme officer reports the number of previously unknown hypertensives who were diagnosed and started on treatment because of the screening. Which term describes this measure?
1. AYield
2. BPredictive value
3. CCoverage
4. DLead time
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Answer: A · Yield
Yield is the amount of previously unrecognised disease that is diagnosed and treated as a result of screening. It depends on the sensitivity of the test, the prevalence of unrecognised disease, whether several diseases are screened for at once, and whether those found positive actually reach diagnosis and treatment. A first screening round gives a higher yield than later rounds, because it picks up the backlog of existing cases. Coverage is the share of the eligible population screened.
- B: Predictive value is the probability that a result is correct.
- C: Coverage is the proportion of the target population screened.
- D: Lead time is the gain in time of diagnosis.
Question 5 · Hard
A district plans to screen 10,000 adults for a disease whose prevalence is 5%, using a test with a sensitivity of 80% and a specificity of 90%. The programme manager must budget for confirmatory tests on all screen-positives. How many of the screen-positives will not have the disease?
1. A100
2. B1,000
3. C1,350
4. D950
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Answer: D · 950
Of 10,000 people, 500 have the disease and 9,500 do not. False positives are (1 - specificity) x non-diseased = 10% of 9,500 = 950. True positives are 80% of 500 = 400, so total screen-positives are 1,350 and the PPV is only 400/1,350, about 30%. Most confirmatory tests will be spent on people who are disease-free, which is the usual cost of screening for an uncommon condition.
- A: 100 is the number of false negatives (20% of 500).
- B: 1,000 takes 10% of the whole population instead of the non-diseased.
- C: 1,350 is all screen-positives, true and false together.
Question 6 · Hard
In a block, tobacco users whose oral cancer is found by screening at an average age of 52 years die of it at 60 years on average. Similar tobacco users in a neighbouring unscreened block develop symptoms and are diagnosed at 55 years on average, and also die at 60 years. What is the true gain in life expectancy from screening?
1. A8 years
2. B3 years
3. CNo gain
4. D5 years
Answer and explanation in LearnFree account
Question 7 · Medium
Two medical officers independently classify the same 200 women as VIA-positive or VIA-negative from photographs of the cervix. They agree on 85% of the women, and the agreement expected by chance alone is 50%. What is the kappa statistic?
1. A0.50
2. B0.85
3. C0.35
4. D0.70
Answer and explanation in LearnFree account
Question 8 · Easy
A district blood centre must choose an assay to screen every donated unit for HIV. Assay A has a sensitivity of 99.8% and a specificity of 98.5%; Assay B has a sensitivity of 97.0% and a specificity of 99.9%. Which assay should the centre choose?
1. AAssay B, for fewer false-positive units
2. BAssay B, for its higher specificity
3. CEither, as both exceed 95% on both counts
4. DAssay A, for its higher sensitivity
Answer and explanation in LearnFree account
Question 9 · Medium
In a screening trial for prostate cancer, tumours found at periodic screening rounds are more often low-grade and slow-growing than tumours that present with symptoms between rounds. Which bias does this illustrate?
1. ALength-time bias
2. BObserver bias
3. CRecall bias
4. DLead-time bias
Answer and explanation in LearnFree account
Question 10 · Medium
For an HbA1c-based screen for diabetes, four cut-offs give these results: A, sensitivity 95% and specificity 60%; B, sensitivity 85% and specificity 80%; C, sensitivity 70% and specificity 92%; D, sensitivity 55% and specificity 98%. If the cut-off is chosen by the Youden index, which one is chosen?
1. AB
2. BD
3. CC
4. DA
Answer and explanation in LearnFree account
Question 11 · Hard
A block runs annual clinical breast examination for women aged 30-65 years. In one year, screening detects 40 breast cancers that are later confirmed. Linkage with the cancer registry shows a further 10 breast cancers diagnosed in screened women within 12 months of a negative examination. What is the estimated sensitivity of the screening examination?
1. AIt cannot be estimated without biopsy of all negatives
2. B80%
3. C25%
4. D20%
Answer and explanation in LearnFree account
Question 12 · Medium
At a cervical screening camp, every woman is examined by two independent tests, and she is referred for colposcopy if either test is positive. Test 1 has a sensitivity of 80% and test 2 a sensitivity of 70%, and the tests are assumed to be independent. What is the net sensitivity of this approach?
1. A80%
2. B56%
3. C94%
4. D75%
Answer and explanation in LearnFree account
Question 13 · Medium
In an active TB case-finding drive in an urban slum, 2,000 adults are screened by symptoms and chest X-ray, and every person is then tested by NAAT, which finds TB in 50. The screen was positive in 40 of the people with TB and in 195 of those without TB. What is the specificity of the screen?
1. A17%
2. B99.4%
3. C90%
4. D80%
Answer and explanation in LearnFree account
Question 14 · Easy
At a factory health camp organised by a district NCD cell, each worker has blood pressure, blood glucose, vision, a chest X-ray and haemoglobin checked at one sitting. What type of screening is this?
1. AMultiphasic screening
2. BSequential screening
3. CHigh-risk screening
4. DOpportunistic screening
Answer and explanation in LearnFree account
Question 15 · Hard
A rapid test for an infection has a sensitivity of 90% and a specificity of 95%. It is used in two places: a referral STI clinic where the prevalence is 20%, and a community survey where the prevalence is 2%. What are the positive predictive values in the clinic and in the community, respectively?
1. A82% and 82%
2. B82% and 27%
3. C27% and 82%
4. D90% and 90%
Answer and explanation in LearnFree account
Question 16 · Easy
A district health officer is deciding which condition to add to a population screening programme. Which feature of a disease most strongly favours screening for it?
1. AIt is rare but always fatal once found
2. BTreatment works as well at any stage
3. CA recognisable latent or early symptomatic stage
4. DIts natural history is poorly understood
Answer and explanation in LearnFree account
Question 17 · Medium
A medical officer screens 5,000 adult tobacco users in her block for oral potentially malignant disorders by oral visual examination. Specialist examination with biopsy, done on everyone, finds a lesion in 100. The visual examination was positive in 80 of these and in 490 of the 4,900 without a lesion. What is the positive predictive value of the visual examination?
1. A99.5%
2. B90%
3. C14%
4. D80%
Answer and explanation in LearnFree account
Question 18 · Medium
A postgraduate presents the ROC curve of a new rapid troponin test to the department. Consider these statements: 1. The vertical axis shows sensitivity. 2. The horizontal axis shows specificity. 3. An area under the curve of 0.5 means the test does no better than chance. 4. A curve closer to the upper left corner indicates a better test. Which statements are correct?
1. A1 and 2 only
2. B2, 3 and 4 only
3. C1, 3 and 4 only
4. D2 and 3 only
Answer and explanation in LearnFree account
Question 19 · Hard
A state starts screening newborns for congenital hypothyroidism; a missed case means preventable intellectual disability, and a positive screen is cheaply confirmed by a venous thyroid test. Three TSH cut-offs are possible: X, sensitivity 99% and specificity 90%; Y, sensitivity 90% and specificity 98%; Z, sensitivity 80% and specificity 99.5%. Which cut-off should be chosen?
1. ANone, as screening is not justified
2. BX, to miss the fewest cases
3. CZ, to recall the fewest babies
4. DY, to balance both properties
Answer and explanation in LearnFree account
Question 20 · Medium
A medical officer reviewing a screening test for anaemia in pregnancy notes four statements about its positive predictive value (PPV): 1. PPV rises as the prevalence of the condition rises. 2. PPV rises as the specificity of the test rises. 3. PPV is independent of the sensitivity of the test. 4. PPV equals 1 minus the negative predictive value. Which statements are correct?
1. A1, 3 and 4 only
2. B1 and 3 only
3. C2 and 4 only
4. D1 and 2 only
Answer and explanation in LearnFree account
Question 21 · Easy
Under NP-NCD, a glucometer-based diabetes screening test that was evaluated in a hospital diabetes clinic is now used by community health officers to screen all adults aged 30 years and above in a rural block, where diabetes is far less common. Its sensitivity and specificity are unchanged. What happens to its positive predictive value?
1. AIt stays the same
2. BIt rises
3. CIt becomes equal to sensitivity
4. DIt falls
Answer and explanation in LearnFree account
Question 22 · Medium
A new rapid card test for falciparum malaria is evaluated against microscopy at a district hospital and found to have a sensitivity of 80% and a specificity of 95%. What is the likelihood ratio of a positive result?
1. A0.75
2. B0.84
3. C16
4. D0.21
Answer and explanation in LearnFree account
Question 23 · Medium
A district screens 10,000 pregnant women for a condition with a prevalence of 2%, using a test with 98% sensitivity and 96% specificity. What proportion of the women who test positive actually have the condition?
1. A33%
2. B98%
3. C2%
4. D96%
Answer and explanation in LearnFree account
Question 24 · Hard
At a district hospital, a 58-year-old diabetic smoker has atypical chest discomfort, and the physician estimates his pre-test probability of coronary artery disease at 20%. A treadmill test with a sensitivity of 80% and a specificity of 90% is positive. What is his post-test probability of coronary artery disease?
1. A20%
2. B80%
3. C67%
4. D89%
Answer and explanation in LearnFree account
Question 25 · Medium
A hospital audit of colorectal cancer reports that patients picked up by a faecal occult blood test survive a mean of 7 years after diagnosis, against 4 years for those who came with symptoms; patients in both groups die of the cancer at about 66 years. Which bias best explains the apparent benefit?
1. ALength bias
2. BSelection bias
3. COverdiagnosis bias
4. DLead-time bias
Answer and explanation in LearnFree account

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Screening: sensitivity, specificity, PPV, NPV

A new rapid test for syphilis is evaluated in 1,000 adults at an STI clinic, of whom 100 have syphilis by the reference test. The rapid test is positive in 90 of the people with syphilis and in 90 of those without it. What is the positive predictive value of the rapid test?

At a PHC NCD clinic, the medical officer lowers the fasting plasma glucose level used to call a screening result positive from 126 mg/dL to 110 mg/dL. What is the expected effect?

A 52-year-old man attends the medical OPD of a CHC for a sprained ankle. The doctor also checks his blood pressure, finds it raised and arranges follow-up. Which type of screening is this?

After a first round of population-based screening for hypertension in a district, the programme officer reports the number of previously unknown hypertensives who were diagnosed and started on treatment because of the screening. Which term describes this measure?

A district plans to screen 10,000 adults for a disease whose prevalence is 5%, using a test with a sensitivity of 80% and a specificity of 90%. The programme manager must budget for confirmatory tests on all screen-positives. How many of the screen-positives will not have the disease?

Two medical officers independently classify the same 200 women as VIA-positive or VIA-negative from photographs of the cervix. They agree on 85% of the women, and the agreement expected by chance alone is 50%. What is the kappa statistic?

A district blood centre must choose an assay to screen every donated unit for HIV. Assay A has a sensitivity of 99.8% and a specificity of 98.5%; Assay B has a sensitivity of 97.0% and a specificity of 99.9%. Which assay should the centre choose?

In a screening trial for prostate cancer, tumours found at periodic screening rounds are more often low-grade and slow-growing than tumours that present with symptoms between rounds. Which bias does this illustrate?

At a cervical screening camp, every woman is examined by two independent tests, and she is referred for colposcopy if either test is positive. Test 1 has a sensitivity of 80% and test 2 a sensitivity of 70%, and the tests are assumed to be independent. What is the net sensitivity of this approach?

In an active TB case-finding drive in an urban slum, 2,000 adults are screened by symptoms and chest X-ray, and every person is then tested by NAAT, which finds TB in 50. The screen was positive in 40 of the people with TB and in 195 of those without TB. What is the specificity of the screen?

At a factory health camp organised by a district NCD cell, each worker has blood pressure, blood glucose, vision, a chest X-ray and haemoglobin checked at one sitting. What type of screening is this?

A rapid test for an infection has a sensitivity of 90% and a specificity of 95%. It is used in two places: a referral STI clinic where the prevalence is 20%, and a community survey where the prevalence is 2%. What are the positive predictive values in the clinic and in the community, respectively?

A district health officer is deciding which condition to add to a population screening programme. Which feature of a disease most strongly favours screening for it?

A new rapid card test for falciparum malaria is evaluated against microscopy at a district hospital and found to have a sensitivity of 80% and a specificity of 95%. What is the likelihood ratio of a positive result?

A district screens 10,000 pregnant women for a condition with a prevalence of 2%, using a test with 98% sensitivity and 96% specificity. What proportion of the women who test positive actually have the condition?