Screening of disease Flashcards
Degree of Freedom calculation
(Column-1) x (Row-1)
Degree of freedom means
A number of factors on which a variable depends
Is false positive or false negative more dangerous?
False negative
Sensitivity is aka?
What does it mean?
Usefulness
It is the ability of a test to detect a diseased person
Basically asking if the person has disease
Positive predictive value
If a person is found to be positive with a test, what is the probability he has the disease
Basically if the person has disease
Specificity and Negative predictive value basically mean
Asking if the person doesnt have disease
Specificity is ability of a test to detect a non diseased person
Negative predictive value is probablity of a person not having the disease if the person tested negative on a test
Formula for sensitivity and specificity
Sensitivity= TP/(TP +FN)
Specificity= TN/ (TN+ FP)
Formula for PPV and NPV
PPV= TP/ (TP + FP)
NPV= TN/ (TN + FN)
Positive likelihood ratio
Negative likelihood ratio
Sensitivity/ (1- Specificity)
(1-sensitivity)/ specificity
Baye’s theorem for PPV
and NPV
PPV= (Sensitivity (Sn) x Prevalence (p))/ (Sn x p) + (1- specificity (Sp)) x (1-p)
NPV=
Sp x (1-p)/ (1-Sn) x p + Sp x (1-p)
What gives the diagnostic power of a test?
Positive and negative predictive value
Relation of prevalence to PPV and False positive?
Prevalence directly proportional to PPV and inversely proportional to False positives
PPV depends on
Prevalence
Sensitivity
Specificity
It doesn’t depend on- Incidence
Prevalence was priorly called as
Pretest/ prior probability
Predictive value was initially known as
Post test probability
If prevalence is more, what happens to PPV, NPV, sensitivity and specificity
PPV increases
NPV decreases
Sensivity and specificity remains constant as they are intrinsic properties of test
Relation between PPV and NPV
Relation between Sensitivity and specificity
PPV is inversely proportional to NPV
Sensitivity is inversely proportional to Specificity
If Sensitivity of a test increases, what happens to True positives, False positives and False negatives
True positives increase
False positives increase
False negatives decrease
For screening tests, we always prefer higher ____
Sensitivity
Because increased sensitivity means decreased FN
Screening is always done in _____ population because ___ is more there
High risk
Prevalence
Prevelance is inversely proportional to FP
Lead time
Time between early diagnosis and usual point of diagnosis which allows us for early treatment, reduced complications and better prognosis
Lead time bias
Patient has no improvement even after early diagnosis
Eg. Rabies- no treatment
Pancreatic cancer- rapid progression
