LC Exam 2: Breast Cancer Screening and Stats

Question 1

Requirement of successful screening programs

Answer 1

Advances time to diagnosis for cancers with consequences

Early treatment is beneficial compared to after sx onset

Question 2

Lead time bias
Length time bias
Overdx bias

Answer 2

Lead time: find earlier, live longer with dz (appear to live longer)
Length time: slower growing -> better prognosis
Overdx: benign natural hx -> better prognosis (wasn’t going to cause trouble in first place)

Question 3

Sensitivity and specificity

Answer 3

Disease No disease
+ Test True + False +
- Test False - True -

Sen: TP/(TP+FN) – Disease column
Spe: TN/(TN+FP) – No disease column

True always on top

Question 4

PPV and NPV

Answer 4

Disease No disease
+ Test True + False +
- Test False - True -

PPV: TP/(TP+FP) = Chance I have dz when test is +
NPV: TN/(TN+FN) = Chance I don’t have dx when test is -

Question 5

SPIN and SNOUT

Answer 5

SPecific tests rule IN (less false positives)

SeNsitive tests rule OUT (higher false positives)

Question 6

More sensitive test has higher incidence of:

False positives or false negatives?

Answer 6

More false positives

Question 7

As prevalence increases what happens to predictive value?

Answer 7

PPV increases as prevalence increases
NPV decreases
(Therefore false negatives more common)

Question 8

Likelyhood ratio

Answer 8

Moves from pretest to posttest probability

Prob of test result in person with disease/
Prob of same test result in person without disease

LR = 1 no impact on likelihood of disease
LR >1 more likely
LR<1 less likely

LR <0.1 = great at ruling out
LR >10 = great at ruling in

Question 9

LR calculation

Answer 9

LR+ = sen/1-spe (positive test result)

LR- = inverse (negative test result)

Question 10

What is relative risk reduction?

Answer 10

Reduction of risk attributable to an intervention compared to a control

RRR=1-RR

RR = [a/(a+b)]/[c/(c+d)]

Question 11

When does OR = RR?

Answer 11

When prevalence is LOW OR=RR

Question 12

What is the absolute risk reduction?

Answer 12

Difference in risk (not proportion) between intervention and control
aka: difference in risk between groups:

ARR= Group1%-Group2% or
c/(c+d) - a/(a+b)

Question 13

NNT

Answer 13

Number of patients that need to be treated or screened for one patient to benefit

NNT = 1/ARR

Question 14

NNS (screen) variation with prevalence?

Answer 14

As prevalence increases, NNS goes down

Easier to find

Question 15

Guideline for screening breast MRI?

Guideline for prophylactic tamoxifan?

Answer 15

lifetime risk of developing breast cancer is >20%

5 yr risk is >3%