Intro to Lab Medicine

Question 1

What is a reference range?

Answer 1

Population mean +/- 2 std deviations

Question 2

Total cholesterol

Answer 2

Good: Below 200
Bad: >240

Question 3

HDL

Answer 3

Good: >60
Bad: <40

Question 4

TG

Answer 4

Good: <150
Bad: >500

Question 5

LDL

Answer 5

Good: <130
Bad: >160

Question 6

Normal blod glucose/A1c

Answer 6

70-99

<5.7

Question 7

Diabetes

Answer 7

> 126

>6.5

Question 8

Tells what fraction of people with disease have a positive test result

Answer 8

Diagnostic sensitivity

TP/ (TP+FN)

Question 9

Tells what fraction of healthy people have a negative test

Answer 9

diagnostic specificty

TN/ (TN +FP)

Question 10

Increasing sensitivity leads to _______ sensitivity

Answer 10

decreasing

Question 11

What is a receiver operator curve?

Answer 11

 Plot of sensitivity vs. 1‐specificity

 Graphical representation of separation

 Measure area under the ROC curve (AUC)

 Higher values for AUC indicate better ability
to SEPARATE the groups of interest

Question 12

If a patient has a positive test result, how liekly is he to have the disease?

Answer 12

PPV =

TP/ (TP+FP)

Question 13

A patient’s test result for a disease is positive. The test is 80% sensitive and 95% specific. How likely is it she has the disease?

Answer 13

TP=80, FP =5, so TP/(TP+FP) = 80/85 or 94%

Question 14

If your patient has a negative test result, how likely is she to not have disease?

Answer 14

TN/(TN+FN)

Question 15

How does prevalence influence predictive values? Higher prevalence? Lower prevalence?

Answer 15

 Disease prevalence in a population has a significant impact on predictive values

 Higher prevalence = fewer FP (more likely to have a TP or disease… AIDS in africa)

 Lower prevalence = fewer FN (More likely to have a true negative)

Question 16

How does prevalence effect PPV?

Answer 16

as prevalence increases PPV increases

Question 17

What are examples of high prevalence?

Answer 17

 HIV‐1 infection prevalence in many sub‐ Saharan countries is in the 20‐30% range of the entire population
 The prevalence of Alzheimer disease increases dramatically with age: 1.5% of people aged 60‐69 have the disease vs. ~40% of those aged ≥90
 Specialist’s practice vs. primary care

Question 18

Newborn screening programs have a high FP which in turn affects PPV…

Answer 18

LOW PPV

Question 19

What are examples of pre-analytical errors?

Answer 19

• Test selection and ordering
– Wrong test, order missed or mis‐transcribed
• Specimen collection
– Wrong type of specimen or container, wrong time
of collection, contamination, insufficient sample • Specimen labeling (identification)
– MOST COMMON
• Transportation to laboratory
– Delays and time‐dependent changes in constituents, loss of sample, hemolysis

Question 20

What are analytical error examples?

Answer 20

 Clerical errors in the laboratory
 Sample identification mix‐ups
 Wrong test performed
 Technical errors (Assay failure, interferences)

Question 21

 Results not communicated
 Results communicated to wrong physician  Wrong results communicated
 Results not interpreted correctly

Are examples of…

Answer 21

Post-Analytical error