Specificity
We have earlier looked at what is normal and the sensitivity of a test.
Let us now look at the specificity of a test (or a sign or a symptom).
What does the specificity of a test translate to?
Specificity is the ability of the test to identify the true negatives- those without the disease.
Who are the true negatives? Persons without the disease and identified by the test as without the disease.
Specificity= True negatives/persons without the disease.
| Test |
Target Disorder |
|
|
| Present | Absent | Total | |
| >= 126 mg/dl (positive) | 215 | 16 | 231 |
| < 126 mg/dl | 15 | 114 | 129 |
| Total | 230 | 130 | 360 |
What is the specificity here? 114/130=87.69%
Points to consider
- Both sensitivity and specificity are functions of the ability of a test to identify the true positives and the true negatives
- Thus both sensitivity and specificity have persons as the denominator-persons with the disease for sensitivity, persons without the disease for specificity. (If we are comparing two tests, then the gold standard test is the denominator)
- As the sensitivity and specificity increase, the number of false negatives and false positives decrease.
- A highly specific test need not be sensitive, a highly sensitive test need not be specific
- Usually a specificity of 95% is considered good (to avoid falsely labeling a person as diseased)
How do you use specificity in the clinical setting?
- If a highly specific test is positive, it rules in the disease (SPIN)
- Why is that? Because a highly specific test has fewer false positives…so, if a highly specific test is positive, it is more likely to rule in the disease!
Remember SNOUT? If a highly sensitive test is negative, it rules out the disease (because a highly sensitive test has fewer false negatives).
Brilliant, isn’t it? We can look at the test result (or sign or symptom) and either rule in or rule out the disease (a filter for further testing) based on the sensitivity and specificity of the test.
Trouble- We need to know the sensitivity and specificity of the test……to apply SNOUT or SPIN!
Caveat: Sensitivity and specificity are functions of the test. They help to rule in or rule out but do not yet establish a diagnosis.
