II. Definitions
- Test Specificity
- Screening Test correctly negative in absence of disease
- Percentage of patients without the disease who have an appropriately negative test for the disease
- A test with high Specificity has few False Positives
- Independent of disease Prevalence in the community
- Specific Tests allow user to RULE IN or confirm a condition (mnemonic "SPin")
- Screening Test correctly negative in absence of disease
III. Calculation
- Test Specificity
- True negative tests per unaffected patients tested
- Expressed as a percentage
- Test Specificity = P(negative test | no disease)
- Where P (A | B) = Probability of A given B
- False Positive Rate
- Test positive despite absence of condition
- False Positive Rate = 1 - Test Specificity
IV. Example: A new Screening Test for Crohn's Disease
- Patients without Crohn's Disease tested: 255
- Patients without Crohn's Disease who have a negative test: 230
- Specificity = 230/255 or 90%
V. Precaution
- Test Specificity can be misleading
- Example
- Condition A is actually present in 150 patients (5%) of the 3000 patients tested
- Therefore 2850 patients do not have condition A
- Test Specificity of 90% would result in a 10% False Positive Rate (of 2850) or 285 patients
- In this case a 90% Test Specificity would result in a False Positive result in 285 patients, when only 150 actually had the condition
- Conclusion
- The lower the Prevalence of disease in the cohort tested, the higher the Test Specificity must be to give a reasonable likelihood of correctness
- Positive Predictive Value may be a more valuable measure as it takes the condition Prevalence into account
- Risk stratifying a group prior to testing can concentrate patients more likely to be positive without missing a significant number
- Example: Limit D-Dimer testing to the intermediate likelihood of Pulmonary Embolism group (based on Wells Score)
- This increases the Prevalence in the tested group and reduces the number of patients with False Positive results
VI. References
- Hennekens (1987) Epidemiology Medicine, p.327-47
- Majoewsky (2012) EM:RAP 12(1): 9-11
- Gates (2001) Am Fam Physician 63(3):513-22 [PubMed]
- MacLean (1996) Med Clin North Am 80(1):1-14 [PubMed]
- Nielsen (1999) Med Clin North Am 83(6):1323-37 [PubMed]
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Related Studies
| Definition (MSH) | Positive test results in subjects who do not possess the attribute for which the test is conducted. The labeling of healthy persons as diseased when screening in the detection of disease. (Last, A Dictionary of Epidemiology, 2d ed) |
| Concepts | Laboratory or Test Result (T034) |
| MSH | D005189 |
| English | False Positive Reaction, False Positive Reactions, Positive Reaction, False, Positive Reactions, False, Reaction, False Positive, Reactions, False Positive |
| Swedish | Falskt positiva reaktioner |
| Czech | falešně pozitivní reakce |
| Finnish | Väärät positiiviset tulokset |
| Russian | LOZHNOPOLOZHITEL'NYE REAKTSII, ЛОЖНОПОЛОЖИТЕЛЬНЫЕ РЕАКЦИИ |
| French | Réactions faussement positives, Tests faussement positifs, Faux positifs |
| Croatian | LAŽNO POZITIVNE REAKCIJE |
| Polish | Wyniki fałszywie dodatnie |
| Norwegian | Falske positive reaksjoner |
| German | Falsch-positive Reaktionen |
| Italian | Reazioni falso-positive |
| Dutch | Reactie, fals-positieve |
| Portuguese | Reações Falso-Positivas |
| Spanish | Reacciones Falso Positivas |
Ontology: Specificity (C0037791)
| Concepts | Quantitative Concept (T081) |
| MSH | D012680 |
| Portuguese | Especificidade |
| Spanish | Especificidad |
| French | Spécificité |
| German | Spezifität |
| English | Specificity, specificity |
| Croatian | SPECIFIČNOST |
| Norwegian | Spesifisitet |
| Italian | Specificità |
| Dutch | Specificiteit |
Ontology: specificity of measurement (C0681901)
| Concepts | Quantitative Concept (T081) |
| English | specificity of measurement |
Ontology: Diagnostic Specificity (C1511884)
| Definition (NCI_NCI-GLOSS) | The frequency with which a test yields a true negative result among individuals who do not have the disease or the gene mutation in question. A test with high specificity has a low false-positive rate and thus does a good job of correctly classifying unaffected individuals. |
| Definition (NCI) | The probability that a test will produce a true negative result when used on non-effected subjects as compared to a reference or "gold standard". The specificity of a test can be determined by calculating: number of true negative results divided by the sum of true negative results plus number of false positive results. |
| Concepts | Quantitative Concept (T081) |
| English | Specificity of Diagnostic Test, Specificity, Diagnostic Specificity, specificity |
