In a village having a population of 1000, we found patients with ceain disease. The results of new diagnostic test on that disease are as follows Test Result Disease Disease Present Absent + 180 400 – 20 400 What is the prevalence of the disease?
Looking at the table, the rows are Test Results (Positive and Negative), and the columns are Disease Present and Absent. The numbers provided are: Test + Disease Present 180, Disease Absent 400; Test - Disease Present 20, Disease Absent 400. Wait, that adds up to 180+400+20+400 = 1000, which matches the population size.
To calculate prevalence, I need the total number of people who have the disease regardless of the test result. So Disease Present would be 180 (true positives) plus 20 (false negatives), totaling 200. So prevalence is 200/1000 = 20%.
Now, checking the options. The correct answer should be 20%. The options aren't labeled here, but the user mentioned that the correct answer is provided. The key here is that prevalence is calculated from the disease present column, not the test results. So the test's accuracy (like sensitivity or specificity) isn't needed here.
The common mistake might be confusing prevalence with incidence or test characteristics. For example, someone might add the true positives and false positives (180 + 400 = 580) and divide by population, but that's the number of positive test results, not the actual disease presence. So the wrong options might be 58%, 20%, 18%, etc.
Another possible error is miscalculating the total disease present. If someone forgot to include the false negatives (the 20), they might get 180/1000 = 18%, which is incorrect. Also, prevalence is a measure of existing cases, not new ones, so incidence isn't relevant here.
The clinical pearl here is to always remember that prevalence is total cases (present) divided by total population. The test results don't affect prevalence calculation directly; they're for sensitivity and specificity. So in a 2x2 table, prevalence is (True Positive + False Negative)/Total Population.
**Core Concept**
Prevalence measures the proportion of a population with a disease at a specific time. It is calculated as **(Number of existing cases) / (Total population) × 100**, independent of diagnostic test results.
**Why the Correct Answer is Right**
In the 2×2 table, the total number of disease **present** cases is **180 (true positives) + 20 (false negatives) = 200**. The prevalence is thus **200/1000 × 100 = 20%**. This calculation focuses solely on the disease status (disease present/absent) and not on test outcomes.
**Why Each Wrong Option is Incorrect**
**Option A:** Likely assumes