9. MTB Step 3 - Sensitivity & Specificity, NPV & PPV, ARR & RRR) Flashcards
Cards Complete:
SENSITIVITY & SPECIFICITY
Does the Sensitivity & Specificity of a test change based on the Prevalence, or Rate of Disease in a community?
NOOOOOO!!!!!
Sensitivity & Specificity start with the Disease
CALCULATION
What is the equation for Sensitivity?
Sensitivity = TP / TP + FN
CALCULATION
What is the equation for Specificity?
Specificity = TN / TN + FP
DEFINITION
What is the definition of Sensitivity?
SENSITIVITY
The likelihood that patients WITH the disease will test POSITIVE
DEFINITION
What is the definition of Specificity?
SPECIFICITY
The likelihood that patients WITHOUT the disease will test NEGATIVE
CALCULATION
Positive Predictive Value (PPV)
PPV = TP / TP + FP
CALCULATION
Negative Predictive Value (NPV)
NPV = TN / TN + FN
CALCULATION
What is the equation for Number Needed to Treat (NNT)?
NNT = 1 / ARR
- where Attributable Rate Reduction (ARR) = (C/C+D) - (A/A+B)*
- where Risk with Treatment = A/A+B*
- where Risk of NO Treatment = C/C+D*
CALCULATION
What is the equation for Number Needed to Harm (NNH)
NNH = 1 / ARI
- where Attributable Risk Increase (ARI) = (C/C+D) - (A/A+B)*
- where Risk with Treatment = A/A+B*
- where Risk of NO Treatment = C/C+D*
- (a.k.a. Attributable Risk)*
SAID ANOTHER WAY
What are (5) different ways of explaining Sensitivity?
- All the people WITH a disease should have a POSITIVE test.
- A NEGATIVE result of a highly sensitive test will EXCLUDE a disease.
- If the test is perfectly sensitive, there will be NO False Negatives.
- A NEGATIVE test rules a disease OUT (SnOut)
- If you HAVE the disease, will you have a POSITIVE test?
SAID ANOTHER WAY
What are (6) different ways of explaining Specificity?
- All the people WITHOUT a disease should have a NEGATIVE test.
- A POSITIVE result of a highly specific test will INCLUDE a disease.
- If the test is perfectly specific, there will be NO False Positives.
- A POSITIVE test rules a disease IN (SpIn)
- If you DON’T have the disease, will you have a NEGATIVE test?
CALCULATION
What is the equation for Absolute Risk Reduction (ARR)?
ARR = (A / A + B) - (C / C + D)
- where the Risk of exposure = A/A+B*
- where the Risk of no exposure = C/C+D*
CALCULATION
What is the equation for Relative Risk Reduction (RRR)?
RRR = ARR / (C / C + D)
where ARR = (A/A+B) - (C/C+D)
CALCULATION
What is the equation for Relative Risk (RR)?
RR = (A / A + B) / (C / C + D)
EXAMPLE
What is the Absolute Risk Reduction (ARR) if the following are true?:
- The Mortality Rate for an Anterior Wall MI with NO treatment is 40% (40 out of 100 will die within first year post-MI).
- The Mortality Rate for an Anterior Wall MI with Angioplasty in first 90 mins of arrival in ED is 20% (20 out of 100 will die within first year post-MI)
ARR = (A / A + B) - (C / C/ + D)
- where Risk with NO treatment = A/A+B*
- where Risk with Angioplasty = C/C+D*
Therefore…
ARR = 40% - 20% = 20%
