RANDOM STEP 2 CK

Question 1

SN is the probability that ____.

Answer 1

a person with the disease will have a positive result aka True Positive (TP) rate.

SN rules out a disease (SNout)!

E.g ELISA for HIV screening

Question 2

SP is the probability that ____.

Answer 2

a person without the disease will test negative aka True Negative rate.

SP “rules in” a disease (SPin)!

E.g. Westernblot for HIV confirmation.

Question 3

In a ROC curve (receiver operating characteristics), TP rate (SN) is plotted on the ___ axis, and FP rate (1-SP) is plotted on the ___ axis.

Answer 3

TP rate (SN) on the y-axis, and

FP rate (1-SP) on the x-axis.

SEE IMAGE

Question 4

What is the conceptual difference between SN and SP versus predictive values (NPV and PPV)?

Answer 4

SN & SP refer to the test characteristics.

Predictive values depend on both to test characteristics as well as to the disease prevalence.

Question 5

Explain PPV.

Answer 5

PPV is the probability that a person who tested positive truly has the disease, DEPENDING on the prevalence of the disease.

↑↑ disease prevalence –> ↑↑PPV

↑ raising test cut-off –> ↑ SP & ↑ PPV.

Question 6

Explain NPV.

Answer 6

NPV is the probability that a person who tested negative truly does not have the disease, DEPENDING on the prevalence of the disease.

↓ disease prevalence –> ↑NPV

↓ raising test cut-off –> ↑ SN & ↑ NPV.

Question 7

True/False? SP and PPV shift in the same direction, and SN and NPV shift in the same direction when the cut-off values of tests are changed.

Answer 7

true (See attached image)

↑ cut-off –> ↓ FP –> ↑ SP & ↑ PPV.

↓ cut-off –> ↑ FP –>↑ SN & ↑ NPV

Question 8

What is the likelihood ratio (LR)?

Answer 8

The likelihood that a given test result would be expected in a patient with the target disorder COMPARED TO the likelihood that that same result would be expected in a patient without the target disorder.

in other words,
-LR +ve shows how much the odds of the disease are increased if the test result is positive.
-LR -ve shows how much the odds of the disease are decreased if the test result is negative.

Question 9

LR + is calculated as __.

Answer 9

LR + = SN/1-SP

Question 10

LR - is calculated as __.

Answer 10

LR - = 1-SN/SP

Question 11

Post-test odds are equal to __.

Answer 11

pre-test odds x LR.

Question 12

The total O2 content of arterial blood (CaO2) is calculated by ___.

Answer 12

CaO2 = Hb-bound O2 + dissolved O2

Bound O2
= (O2 carrying capacity of Hb) (SaO2)
= (1.34 x Hb) (SaO2)
=1.34 x Hb x SaO2

Dissolved O2
= PaO2 + *Sol O2
* solubility coefficient of O2
=PaO2 x 0.0031

CaO2 = (1.34 x Hb x SaO2) + (PaO2 x 0.0031)

Question 13

Sr. calcium corrected for albumin

Answer 13

Corrected ca2+ (mg/dL) =
measured total Ca2+ (mg/dL) + 0.8 (4.0 - sr. albumin [g/dL]),
where 4.0 represents the average albumin level.

also as below

adjusted Ca (mmol/L) =
total Ca (mmol/L) + 0.02 (40 - albumin (g/L).

Question 14

What is the next best step for a patient with hypocalcemia (albumin corrected) with low or normal PTH?

Answer 14

Check sr. Mg levels;

If low –> hypoCa2+ d/t Mg2+ deficiency

If normal –? Possible hypoPTH or defect in Ca2+ sensing receptors.

Question 15

What is the next best step for a patient with hypocalcemia (albumin corrected) with high PTH?

Answer 15

Check bun & creatinine –> Renal f.

Renal function normal –> check Vit. D

Vit. D low-> hypoca2+ & compensatory HyperPTH

Vit. D normal –> PseudohypoPTH or Ca2+ deficiency (rare).