Key Formulas

Question 1

Diagnostic Sensitivity Formula

Answer 1

true positives/(true positives + false negatives)
Also known as: Recall or True Positive Rate

Question 2

Diagnostic Specificity Formula

Answer 2

true negatives/(true negatives + false positives)
Also known as: True Negative Rate

Question 3

Positive Predictive Value (PPV)

Answer 3

true positives/(true positives + false positives)
Also known as: Precision

Question 4

Negative Predictive Value (NPV)

Answer 4

true negatives/(true negatives + false negatives)

Question 5

What question does sensitivity answer?

Answer 5

If a person has the disease, how likely is the test to be positive?

Question 6

What question does specificity answer?

Answer 6

If a person does NOT have the disease, how likely is the test to be negative?

Question 7

What question does PPV answer?

Answer 7

If the test is positive, how likely is it that the person actually has the disease?

Question 8

What question does NPV answer?

Answer 8

If the test is negative, how likely is it that the person truly does NOT have the disease?

Question 9

Creatinine Clearance (CrCl) — 24-hour urine method formula (SI units)

Answer 9

CrCl (mL/min) = UV/ST

U = urine creatinine concentration (μmol/L)
V = urine volume (mL)
S = serum creatinine concentration (μmol/L)
T = collection time (min)
For 24h urine: T = 1440 min

Question 10

Corrected Creatinine Clearance (for Body Surface Area) formula

Answer 10

Corrected CrCl = CrCl x 1.73/BSA