POC Infectious diseases + Stats

Question 1

What is measured in HIV tests? (3)

Answer 1

HIV antigens
HIV RNA
Antibodies

Question 2

What do POC rapid HIV test for?

Answer 2

Antibodies
- indicator of body’s response to the virus

Question 3

Why do we use POC HIV testing in labour/delivery?

Answer 3

It is possible to reduce HIV transmission to the infant if HIV status is known
- provide more rapid results than lab tests and possibly more accurate than ELISA (more expensive though)

Question 4

What are types of POC HIV tests available?

Answer 4

• Lateral flow tests (similar to pregnancy, RATs)
• Flow-through tests

Question 5

What is the INSTI test?
Detects?
What kind of test?
What does it use from your body?

Answer 5

HIV test
Flow through test
Detects presence of HIV-1/HIV-2 antibodies
- uses blood (at home), serum or plasma

Question 6

What are the steps of a flow-through test?

Answer 6

1. Analyte capture molecules: HIV antigens
   - Located at the bottom of the test (to capture)
2. Specimen: usually blood
   - Diluted in specific solution –> flows through membrane toward capture molecule
3. Analyte molecules: HIV antibodies
   - If present, will bind to capture molecules (HIV antigens)
4. Detection molecules: specific solution, added last
   - If antibodies bind to antigens, detection molecules will bind captured analyte and allow visual detection (dot)

Question 7

What do tests for strep throat (pharyngitis) test for?
What kind of assay?

Answer 7

Group A streptococcus
Lateral flow assays

Question 8

What symptoms can indicated bacterial pharyngitis rather than viral? Need to confirm?

Answer 8

• Under 14
• Enlarged, tender lymph nodes
• Fever
• Absence of cough

Confirm with microbial testing + throat culture

Question 9

Are pharyngitis rapid antigen detection better at ruling in or ruling out strep throat

Answer 9

Better at ruling in
- high specificity (SPIN)

Question 10

What do the following tests test for COVID-19?
PCR
Rapid antigen
Serological tests

Answer 10

PCR
- Viral RNA

Rapid antigen
- Viral antigens

Serological tests
- Measure antibodies to show past infection

Question 11

When will the tests start to work?
PCR
Rapid antigen
Serological tests

Answer 11

PCR
- can detect sooner
- BUT can stay positive after no longer infectious

Rapid antigen
- Can produce FALSE NEGATIVES early on

Serological tests
- used detect host response to injection a week after symptoms

Question 12

How to improve sensitivity in COVID tests

Answer 12

• swab both cheeks, back of tongue or throat, then both nostrils
• no food/drink/smoking/meds 30 minutes before test

Question 13

What are reasons for false positives for COVID-tests?

Answer 13

1. Acidic foods/bev
2. Not using buffer, using water drinks
3. Leaving test too long
4. Lot-specific devices

Question 14

Define sensitivity

Answer 14

Probability of getting a POSITIVE result WITH disease

Question 15

Define specificity

Answer 15

Probability of getting a NEGATIVE result WITHOUT disease

Question 16

Define false positive. Equation?

Answer 16

Getting an incorrect positive result WITHOUT the disease
1-specificity

Question 17

Define false negative. Equation?

Answer 17

Getting an incorrect negative result WITH disease
1-sensitivity

Question 18

Define positive predictive value

Answer 18

Probability of actually HAVING the disease AMONG people with a POSITIVE test

Question 19

Define negative predictive value

Answer 19

Probability of NOT having the disease AMONG people with a NEGATIVE test

Question 20

Low sensitivity leads to..
high false negative/positive rates?

Answer 20

high False negative

Question 21

Low specificity leads to..
high false negative/positive rates?

Answer 21

high false positive

Question 22

What do positive and negative predictive value highly dependant on?

Answer 22

Prevalence of the disease

Positive predictive value
- HIGHER with high prevalence

Negative predictive value
- LOWER with high prevalence

Question 23

Define likelihood ratio

Answer 23

The probability of a particular test result for a person WITH disease
divided by
WITHOUT disease

Question 24

Define likelihood ratio for a positive test result.
Equation for LR+
How is it expressed as?

Answer 24

The probability of a positive test result for a person WITH disease
divided by
positive WITHOUT disease

Equation: Sensitivity/(1-specificity)
OR
True positive rate/false positive rate

Expressed as a decimal

Question 25

How to phrase likelihood ratios

Answer 25

You are X more likely to get a positive result of this test if you have this disease than without the disease

Question 26

What are likelihood ratios not impacted by?

Answer 26

Prevalence

Question 27

How to interpret LR+ ratios
what is considered a high diagnostic value?
What does it mean if you have a high value?

Answer 27

10+ considered high
- The higher the LR+, the more likely you are of having that disease if you test positive

Question 28

What does a lower cut off mean: positive/negative
Sensitivity + specificity (high/low)

Answer 28

Low = negative
High sensitivity
Low specificity

• lots of false positives

Question 29

What does a higher cut off mean: positive/negative
Sensitivity + specificity (high/low)

Answer 29

High = positive
Low sensitivity
High specificity

• lots of false negatives

Question 30

What does dichotomous mean? Are most values dichotomous or continuous

Answer 30

Disease present or absent

Continuous

Question 31

Which graphs show tests better? What are on the axis?

Answer 31

X-axis: False positive rate vs (1-specificity)
Y-axis: True positive rate (sensitivity)

Higher AUC
- shoulder closer to y-axis