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2.11

Salmonella

# 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)