RCE Formulas

Question 1

Stool Osmolar Gap

Answer 1

290 - 2 (stool Na + stool K)
Normal = 50-100
Low gap = secretory = cholera, ETEC, VIP, gastrinoma, non osm laxative
High gap = osmotic = celiac, lactose, lactulose, chronic pancreatitis, Whipple’s, osmotic laxative abuse

Question 2

SAAG

Answer 2

Serum albumin - ascites albumin
SAAG > 11 = transudative = portal HTN
SAAG <11 = exudative = TB, pancreatitis, malignancy, nephrotic syndrome

Question 3

Ascites salt restriction no diuretics

Answer 3

Urine Na < 78 = compliant, needs diuretic if weight gain

Urine Na > 78 = non compliant, needs diuretic

Question 4

Ascites salt restriction with diuretics

Answer 4

Urine Na <78 = diuretic resistant, increase dose
Urine Na > 78 and gaining weight = non compliant
Urine Na > 78 and weight okay = compliant, continue course

Question 5

Urine Anion Gap

Answer 5

• Measure when you have a NAGMA
• UAG = UNa + UK - UCl
• UAG + = RTA
• UAG - = gut

Question 6

Volume infusate for hyponatremia

Answer 6

Volume = TBW x (desired Na-current Na)/513

• 513 for hypertonic, 154 for NS, 130 for RL
• TBW = 0.5 x wt (F), 0.6 x wt (M)

Question 7

Water deficit for hypernatremia

Answer 7

Water deficit = Na - 140/140 x TBW

- Correct at 0.5/hr with free water for chronic

Question 8

FeNa

Answer 8

Urine Na x Serum Cr / Serum Na x Urine Cr
FeNa<1 pre renal
>1 ATN
>4 post renal

Question 9

RSBI

Answer 9

Rapid shallow breathing index
RR/tidal volume
RSBI >105 = failed extubation

Question 10

Osmolar gap

Answer 10

Serum Osm - Calc Osm

Calc Osm = 2Na + Gluc + BUN + 1.25ETOH

Question 11

A-a gradient

Answer 11

(150 - PaCO2/0.8) - PaO2
Normal is (Age/4) + 4
Wide gap - V/Q mismatch (improves with 100% FiO2), shunt (doesn’t completely improve with 100% FiO2), diffusion
Normal gap - hypoventilation, low FiO2