Coupon Accepted Successfully!



Screening denotes the search for unrecognized disease or defect in apparently (or outwardly) healthy persons by the application of rapid diagnostic tests, examinations or procedures. The main types of screening are
  1. Mass screening:- When all members of a population are screened for disease it is called mass screening. This is very costly and the yield of cases is usually too small to warrant such a screening procedure.
  2. High-risk screening:-or selective screening refers to the situation where tests are offered only to those individuals who are at high risk of developing a specific disease. This makes the screening process more focused and reduces the overall costs.
  3. Multiphasic screening:- done in phases for e.g. To know the TB status in a community first Montour test is done then X-ray, finally sputum test done in phases.
  1. Likelihood ratio - provides an estimate of how much a test result will change the probability of having a disease. It can be positive or negative. Positive likelihood ratio indicates the magnitude of increase in the probability of the disease in view of a positive test result while a negative like hood ratio indicates a decrease in probability with a negative test.
  1. OPAR Score = (Odds that positive are being affected) it’s a ratio of true positive to false positive.
    1. Please remember there are two types of Bias in screening Lead Time bias and Length bias.
    2. Prevalence is directly proportional to true positivity, so when prevalence increases true positivity increases and when prevalence decreases false positivity increases. (AIIMS Nov’08)
    3. Positive Predictive value of test a Prevalence with sensitivity and specificity being constant
    4. Bayes' theorem: As the prevalence of a disease increases, the positive predictive value (PPV) of the test increases and its negative predictive value decrease.
  1. Combination of two or more test.
    1. In Series: Test 1 - positive result -Test 2 → positive result - final result positive [one negative a.  result is negative] specificity and positive predictive value of the combined tests.
    2. In parallel: Test 1 + Test 2 independently -result positive if anyone is positive [one positive result is positive sensitivity of the combined tests.
Series Test 1 Test 2 Final  
  + + + ST ↓
  - + - SP↑
  + - - PPV ↑
Parallel + - + ST ↑
  - + + SP ↓

Extra Edge: Prevalence of the disease has definitely has a bearing on PPV.  The most important measure in a screening test is its sensitivity.  Sensitivity and specificity are inversely proportional.

Screening tests: important points
  1. Test in series- increases specificity.
  2. Test in parallel- increases sensitivity.
  3. Prevalence of the disease has definitely has a bearing on PPV. The most important measure in a screening test is its sensitivity. Sensitivity and specificity are inversely proportional.
  4. Cut off point of screening: test
    1. Disease prevalence High Prevalence - Low cut off is selected to ↑ Sensitivity.
    2. Lethal disease - [Ca Cx, Breast Ca] & Treatment available; ↑ sensitivity is desired.
    3. When T/T does not markedly alter outcome be more specific, limit false +ve.

Test Your Skills Now!
Take a Quiz now
Reviewer Name