Formulas

Question 1

Lights Criteria

Answer 1

PF/Serum ( Protein ) > 0.5 = Exudative
PF/Serum (LDH) > 0.6 = Exudative

Question 2

ARDS Equation

Answer 2

Pa02/Fi02

100-200mg hg = Moderate to severe

< 100mg Hg is SEVERE

Question 3

1. Formula to calculate plasma osmolity?
2. How to calculate for Osmol gap?

Answer 3

2Na + Glucose/18 + BUN/2.8

Measured - Calculated > 10 then there is a gap.

Question 4

What is sensitivity?

Answer 4

(TP/All the people with disease) = TP/(TP+FN)

Question 5

What is specificity?

Answer 5

(TN/All the peole without Disease) = TN/(TN+FP)

Question 6

1. What is the Positive Predictive Value?

What is its relation to Prevalence?

What is its relation to Specificity?

Answer 6

1. The proportion of people who test positive who actually have the disease.
   TP/ All the people with a positive test result =
   TP/(TP +FP.)
2. As Prevalence increases the PPV increases and as Prevalence decreases the PPV decreases.
3. The more specific a test is a catching all the people without the disease the higher the PPV.

Question 7

1. What is Negative Predictive Value?

Answer 7

TN/ All the people with a negative test results

TN/(TN+FN)

Question 8

“How much more likely is the exposed person going to get the disease compared to the nonexposed. This is?

Answer 8

Relative risk.

Incidence rate of exposed / the incidence rate of NON-exposed group. Used in Cohort study where you can get incidence.

Question 9

Absolute Risk

Answer 9

The absolute difference in rates

Experimental group events - Control group events

ARR = EER-CEF

Question 10

Relative Risk

Answer 10

RRR = (EER - CER)/ CER

Question 11

Odds ratio

Answer 11

AD/BC

Question 12

1. Number Needed to Treat
2. Number Needed to Harm

Answer 12

1. Number of patient needed to receive a treatment for one additional patient to benefit

NNT = 1/ARR

2. Number of patients needed to receive a treatment for one additional patient to be harmed

NNH = 1/ARR

Question 13

The ratio of the probability of developing a disease with ar risk factor present to the probability of developing the disease without the risk factor present?

Answer 13

RR = EER/ CER

Question 14

1. LR +
2. LR -

Answer 14

The measure of the odds of having a disease independent of the disease prevalence.

LR + = Sensitivity/ (1-Specificity)

LR - = (1- Sensitivity)/ Specificity

Question 15

Stool Osmolalarity Forumula?

With an Osmlarity 250-400 and a Gap of:
< 50?
50-125?
>125?

Stool Omlolarity
< 250?
290?

Answer 15

Measured Stool Osmolarity - 2x (Na + K)

Osmolarity 250-400
1. Gap < 50 = Secretory Diarrhea /watery
Secretory laxative use Saline laxatives, Senna,
Bisacodyl
Crohns, Microscopic Colitis, Vipomas,
Hyperthyroidism

2. Gap 50-125 = Indeterminate
3. Gap > 125 = Osmotic Diarrhea osmols in stool
   Laxative with Unmeasured Solute
   (Magnesium, Lactulose, Sorbitol)

• Being on Keto Gives me Osmotic Diarrhea

<250 (Stool Mixed with Water)
290 (Normal )
>400 (Mixed with Urine Concentrated)

Colonoscopy - Can Show Melanosis Coli in Chronic Laxative Abuse

Question 16

Stool Osmol Gap

Answer 16

290 – (2 × [stool sodium + stool potassium])

A gap < 50 mOsm/kg (50 mmol/kg) suggests Secretory diarrhea

A gap >100 mOsm/kg (100 mmol/kg) suggests Osmotic diarrhea