II. Definitions

  1. Test Specificity
    1. Screening Test correctly negative in absence of disease
    2. A test with high Specificity has few False Positives
    3. Independent of disease Prevalence in the community
    4. Specific Tests allow user to RULE IN or confirm a condition (mnemonic "SPin")

III. Calculation

  1. Test Specificity
    1. True negative tests per unaffected patients tested
    2. Expressed as a percentage
    3. Test Specificity = P(negative test | no disease)
      1. Where P (A | B) = Probability of A given B
  2. False Positive Rate
    1. Test positive despite absence of condition
    2. False Positive Rate = 1 - Test Specificity

IV. Example: A new Screening Test for Crohn's Disease

  1. Patients without Crohn's Disease tested: 255
  2. Patients without Crohn's Disease who have a negative test: 230
  3. Specificity = 230/255 or 90%

V. Precaution

  1. Test Specificity can be misleading
  2. Example
    1. Condition A is actually present in 150 patients (5%) of the 3000 patients tested
    2. Therefore 2850 patients do not have condition A
    3. Test Specificity of 90% would result in a 10% False Positive Rate (of 2850) or 285 patients
    4. In this case a 90% Test Specificity would result in a False Positive result in 285 patients, when only 150 actually had the condition
  3. Conclusion
    1. The lower the Prevalence of disease in the cohort tested, the higher the Test Specificity must be to give a reasonable likelihood of correctness
    2. Positive Predictive Value may be a more valuable measure as it takes the condition Prevalence into account
    3. Risk stratifying a group prior to testing can concentrate patients more likely to be positive without missing a significant number
      1. Example: Limit D-Dimer testing to the intermediate likelihood of Pulmonary Embolism group (based on Wells Score)
      2. This increases the Prevalence in the tested group and reduces the number of patients with False Positive results

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Related Studies

Ontology: False Positive Reactions (C0015563)

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
French Réactions faussement positives, Tests faussement positifs, Faux positifs
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
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