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Exercise: Calculating test sensitivity and specificity from experimental results

You are now going to be provided with the results of the Salmonella IMS-PCR experiment to give you the opportunity to calculate test sensitivity and specificity. Please access the PDF file provided (in the downloads section below) below to record results in and work out your calculations on.

Pork samples were presented to the person carrying out the test simply as number coded samples (1-20) without any further details (i.e. they were tested blind). The IMS-PCR findings are provided as images within the PDF file, and the determined test result (positive or negative) based on the IMS-PCR analysis recorded in the second column of the table below - the actual or true Salmonella status of all test samples are provided in the 3rd column.

Firstly, you need to decide whether each result represents a:

  • True positive
  • True negative
  • False positive
  • False negative

and record this information in the final column of the table.

Sample No. IMS-PCR result Salmonella status Test result category
1 Pos Pos  
2 Neg Neg  
3 Neg Neg  
4 Neg Pos  
5 Neg Neg  
6 Pos Pos  
7 Neg Neg  
8 Pos Pos  
9 Neg Neg  
10 Pos Pos  
11 Pos Pos  
12 Pos Pos  
13 Pos Pos  
14 Neg Neg  
15 Neg Neg  
16 Neg Neg  
17 Pos Pos  
18 Neg Neg  
19 Pos Pos  
20 Neg Neg  

Secondly, you need to count up the number of samples in each test result category (i.e. positive or negative) and insert these figures in the summary table provided.

Now you are in a position to calculate the detection sensitivity and specificity of the IMS-PCR method used based on the following equations:

Sensitivity = no. of true positives / (no. of true positives + no. of false negatives)

Specificity = no. of true negatives / (no. of true negatives + no. of false positives)

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This article is from the free online course:

Tackling Global Food Safety

Queen's University Belfast