Discuss the advantages and limitations of using a test with HIGH SENSITIVITY as screening test with an example. Need answer from my latest edition of park psm 28th
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Advantages and Limitations of a High-Sensitivity Test as a Screening Test
(Source: Park's Textbook of Preventive and Social Medicine, 28th Edition)
Definition of Sensitivity (Park's PSM)
"Sensitivity is defined as the ability of a test to identify correctly all those who have the disease, that is, 'true-positives'."
Formula: Sensitivity = a / (a + c) × 100
Where:
- a = True positives (diseased + test positive)
- c = False negatives (diseased + test negative)
A 90% sensitivity means 90% of diseased persons will give a true-positive result, and the remaining 10% will give a false-negative result.
Advantages of a High-Sensitivity Test as a Screening Test
1. Few False Negatives
Park's states directly:
"A screening test which is very sensitive has few false negatives. The lower the sensitivity, the larger will be the number of false negatives."
This means almost all truly diseased persons in the population are correctly identified and not missed.
2. Prevents Missed Cases and "False Reassurance"
Park's explains the harm of false negatives:
"The patient with a 'false-negative' test result might ignore the development of signs and symptoms and may postpone treatment. This could be detrimental if the disease in question is a serious one and the screening test is unlikely to be repeated within a short period of time."
A high-sensitivity test avoids this danger by ensuring diseased individuals are not wrongly reassured they are disease-free.
3. Ideal for Serious, Treatable Diseases
For conditions where missing a case has grave consequences (e.g., cancer, HIV, active TB), high sensitivity is preferred so that the screening net catches everyone who may be diseased.
4. Useful in Combination Testing (Serial/Sequential Testing)
Park's describes the classic syphilis example:
"Syphilis screening affords an example whereby all screenees are first evaluated by an RPR test. This test has high sensitivity, yet will yield false positives. However, all those positive to RPR are then submitted to FTA-ABS, which is a more specific test, and the resultant positives now truly have syphilis."
High-sensitivity tests are used as the first-line screen in a two-stage approach - cast the wide net first, then confirm with a specific test.
Limitations of a High-Sensitivity Test as a Screening Test
1. Inverse Relationship with Specificity
Park's states clearly:
"Sensitivity and specificity are inversely related. That is, sensitivity may be increased only at the expense of specificity and vice versa."
So a test cannot be both 100% sensitive and 100% specific simultaneously.
2. More False Positives (Low Specificity)
A high-sensitivity test will necessarily have lower specificity, which means it will produce more false positives - labelling healthy persons as diseased. Park's describes the harm:
"False-positives means that patients who do not have the disease are told that they have the disease. In this case, normal healthy people may be subjected to further diagnostic tests, at some inconvenience, discomfort, anxiety and expense - until their freedom from disease is established."
Park's further notes:
"False-positives not only burden the diagnostic facilities, but they also bring discredit to screening programmes."
3. Predictive Value of a Positive Test Falls in Low-Prevalence Populations
Park's makes this important point:
"The more prevalent a disease is in a given population, the more accurate will be the predictive value of a positive screening test. The predictive value of a positive result falls as disease prevalence declines."
This means even a high-sensitivity test generates many false positives (poor positive predictive value) when used in a population where the disease is rare.
Illustrative Example from Park's PSM
Gram-Stained Cervical Smear for Gonorrhoea
Park's provides Table 7, comparing the performance of a Gram-stained cervical smear test (sensitivity 50%, specificity 90%) at three prevalence levels:
| Prevalence | Positive Predictive Value |
|---|---|
| 5% | 21% (79% are false positives) |
| 15% | 47% |
| 25% | 63% |
Park's commentary:
"In female populations in which the gonorrhoea is low (5% prevalence), only 21% of patients with positive results really have gonorrhoea; the remaining 79% have false-positive results. Furthermore, as the sensitivity of this test is only 50%, half of the cases are not detected, which greatly reduces the impact of the detection programme on disease transmission."
(Note: This example also shows that a test with both low sensitivity AND lower specificity performs poorly - reinforcing why ideal screening tests need high sensitivity AND reasonable specificity together.)
Key Take-Home Points (Park's PSM)
| Property | High Sensitivity Test |
|---|---|
| False negatives | Few - good for screening |
| False positives | More - limitation |
| Best use | First-stage screening for serious diseases |
| Companion test | Followed by a high-specificity confirmatory test |
| Ideal scenario | High prevalence population (improves PPV) |
| Risk if used alone | Overloads diagnostic facilities, causes anxiety |
Park's concludes:
"In fact, no screening test is perfect, i.e., 100 per cent sensitive and 100 per cent specific."
The practical approach recommended by Park's is combination testing - use a high-sensitivity test (e.g., RPR for syphilis) to screen everyone, then confirm positives with a high-specificity test (e.g., FTA-ABS) to eliminate false positives.
- Park's Textbook of Preventive and Social Medicine, 28th Edition, pp. 158-160
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