Avian influenca - Test 2

In a next step you test 50,000 chickens from an avian influenza surveillance programme with the new ELISA you just evaluated (SE = 98.0%, SP = 97.5%).

How many ELISA-positive results do you expect if the true prevalence of chickens with measurable antibodies against AI virus in this population is 10%. How many of these Elisa-positive test results will be false-positive?

iDevice icon Complete the contingency table

Complete the table below to investigate. Calculate the apparent prevalence as well as both the positive and negative predictive values in this population.

Type the values (numbers only) into the contingency table. Use the calculator if required.

  AI +
AI - Total
ELISA +

ELISA -

Total

50000

 

 

 

 

 

 

 
Calculator:

 
  

iDevice icon Calculate sensitivity, specificity and predictive values
Type in the values (round to one number after decimal point, e.g. 99.9; if no decimal just type in full number, e.g. 100):

The true prevalence is %. Thus birds are infected (AI+) and birds are non-infected (AI-).

The test sensitivity is 98%, thus the number of true positive is . Consequently, there are false negative birds.

The test specificity is 97.5%, thus the number of true negative is . Consequently, there are false positive birds.

Finally, there are ELISA+ and ELISA- birds.

We can check that there are in total birds:

  • The apparent prevalence (AP) = %
  • The positive predictive value (PPV) = %
  • The negative predictive value (NPV) = %

  

iDevice icon Discussion and interpretation
What do the numbers tell you? Try to explain in your own words.
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