Last week’s clinical informatics practice question of the week reviewed the AHRQ’s Quality Indicators. This week we’ll be moving on from the qualitative and getting into the quantitative. Yippee!

Specifically, we’ll be discussing sensitivity and specificity.

**Where Does this Fit Within the Core Content?**

This week, we’ll be staying in the CDS and Care Process Improvement area. Sensitivity, specificity, and other test characteristics fall under the topic of decision science, as you can see below.

**2. Clinical Decision Making and Care Process Improvement**

2.1.1. The nature and cognitive aspects of human decision making

2.1.2. Decision science

2.1.2.1. Decision analysis

2.1.2.2. Probability theory

2.1.2.3. Utility and preference assessment

2.1.2.4. Cost effectiveness analysis

**2.1.2.5. Test characteristics (e.g., sensitivity, specificity, predictive value)**

2.1.3. Application of clinical decision support

2.1.4. Transformation of knowledge into clinical decision support tools

2.1.5. Legal, ethical, and regulatory issues

2.1.6. Quality and safety issues

2.1.7. Supporting decisions for populations of patients

If you recall, **sensitivity** is the true positive rate. It’s the likelihood that those that receive a positive test result have the disease.

Meanwhile, **specificity** is the true negative rate. It’s the likelihood that those that receive a negative test result do not have the disease.

We’re not going to include the formulas before the question. However, if you’re not sure where to begin they are available in the answer and explanation section. Just scroll down and you should find them.

So whip out your calculators and let’s get into this week’s practice question.

## Question

400 patients take a diagnostic test. There are:

- 180 True Positives
- 8 False Positives
- 190 True Negatives
- 22 False Negatives

What is the sensitivity and specificity of this test?

A. Sensitivity: 89.62; Specificity: 95.74;

B. Sensitivity: 95.74; Specificity: 89.62;

C. Sensitivity: 89.11; Specificity: 95.96;

D. Sensitivity: 95.96 ; Specificity: 89.11;

## Answer and Explanation

The formulas for sensitivity and specificity are as follows:

Sensitivity = (true positives) / (true positives + false negatives)

Specificity = (true negatives) / (true negatives + false positives)

——

Therefore the sensitivity is (180) / (180 + 22) = 180/202 = 89.11

And the specificity is (190) / (190 + 8) = 190/198 = 95.96

—

The other distractor answers are the positive predictive value and the negative predictive value.

The PPV is the true positives divided by the sum the true positives and false positives. The PPV is 95.74.

The NPV is the true negatives divided by the sum of the true negatives and false negatives. The NPV is 89.62.

Therefore, the correct answer is **C. Sensitivity: 89.11; Specificity: 95.96;.**

## Recommended Reading and Resources

Lalkhen and McCluskey. Clinical tests: sensitivity and specificity. Continuing Education in Anesthesia, Critcal Care & Pain. (2009) 8 (6): 221-223 doi: 10.1093/bjaceaccp/mkn041.

http://ceaccp.oxfordjournals.org/content/8/6/221.full#cited-by

## Further Learning

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