Electrolytes

Na+ (corrected for hyperglycemia):

• Corrected Na+ = measured Na+ + [(glucose – 100) x 0.024]

Ca2+ (corrected for hypoalbuminemia):

• Corrected Ca2+ = [(4 – serum albumin) x 0.8] + measured Ca2+

Free Water Deficit:

• Water deficit = wt (kg) x k x [(plasma Na+ / 140) – 1]; where k = 0.5 for males and 0.4 for females

Osmolality:

• Calculated Osm = (2 x Na+) + (glucose / 18) + (BUN / 2.8) + (EtOH / 4.6) 
• normal: 270–290
• Osm gap = measured Osm – calculated Osm {normal < 10}
• >10 is abnormal: caused by renal failure, methanol, ethylene glycol, sorbitol, mannitol, isopropanol, radiocontrast dye

Anion Gaps:

• Serum AG = [Na+] – [Cl-] – [HCO3] {normal 10-14}
• Corrected AG (for hypoalbuminemia)
• Corrected AG = serum AG + [(4 – serum albumin) x 2.5]
• ∆AG = (AG – 12) + HCO3 {normal 23–30}.
• ∆AG >30: concomitant metabolic alkalosis (excessively high HCO3).
• ∆AG <23: concomitant non-AG metabolic acidosis (excessively low HCO3).
• Urine AG = U[Na+] + U[K+] – U[Cl-]
• NH4+ is the major unmeasured cation, so a strongly negative UAG suggests high urine NH4+.
• Urine AG <0: GI HCO3 loss.
• Urine AG >0: Renal HCO3loss (RTA). 

Body Fluid / IV Fluids

Body fluid composition:

• Total Body Water = 0.6 x wt (kg) for males 0.5 x wt (kg) for females
• Extracellular fluid (ECF) = 0.2 x wt (kg)
• Intravascular: 1/3 ECF 
• Interstitial: 2/3 ECF
• Intracellular fluid (ICF) = 0.4 x wt (kg)

IVF and tonicity:

Plasma	142	4	104	27	29	306
NS	154	-	154	-	-	308
D5W	-	-	-	-	-	278
D5 1⁄2NS	77	-	77	-	-	421
1 amp NaHCO3	50	-	-	50	-	100
20 mEq KCl	-	20	20	-	-	40

• e.g. 1⁄4NS + 20 mEq KCl + 1⁄2 amp NaHCO3 = (308/4) + 40 + (100/2) = 167 mOsm/L, or roughly equivalent in tonicity to 1⁄2NS

IVF effect on plasma Na:

• ∆Na per liter IVF given = [NaIVF(mEq/l) + KIVF(mEq/l) – Naserum (mmol/l)] / [Total Body Water + 1]

Renal

Creatinine Clearance:

• Estimated CrCl (Cockcroft-Gault Equation)* = [(140 – age) x (wt in kg)] / [serum Cr x 72]
• *multiply by 0.85 for females
• {normal 100–125 ml/min (M) or 85–105 ml/min (F)}
• Measured CrCl = [urine Cr x urine volume (ml/24hr)] / [serum Cr x 1440 (min/24hr)]

Fraction Na excretion:

• FENa = [(urine Na+ / serum Na+) x 100] / [(urine Cr / serum Cr)]
• <1% suggests pre-renal
• FENa interpretable only in oliguric states (UOP < 400 cc/day)

Transtubular Potassium Gradient:

• TTKG = (urine K+ / serum K+) / (urine Osm / serum Osm)
• With hyperkalemia:
• < 7 suggestive decreased aldosterone activity.
• > 7 suggestive effective volume depletion with normal aldosterone activity. 

Hemodynamics

Cardiac Output:

• CO = HR x stroke volume
• CI = CO / BSA (in m2); where BSA= √([ht (cm) x wt (kg)] / 3600)
• Fick Technique: CO =  estimated O2 consumption or VO2 (ml/min)* / arteriovenous O2 difference** 

Resistance:

• SVR = [(MAP – CVP) / CO] x 80 
• {normal 700–1600 dynes*s/cm5}
• PVR = [(MPAP – PCWP) / CO] x 80 
• {normal 20–120 dynes*s/cm5}

Pressure:

• Pulse Pressure = SBP – DBP
• MAP = [SBP + (DBP x 2)] / 3

Pulmonary

Alveolar-arterial O2 gradient:

• A–a O2 gradient = [FiO2 x (pAtm – pH2O)] – (pCO2 / R) – pO2
• = [FiO2 x (760 – 47)] – (pCO2 / 0.8) – pO2
• = 150 – (pCO2 / 0.8) – pO2
• {normal 5–25 or age/3} *at sea level on RA

Statistics

	Disease (+)	Disease (–)
Test (+)	A True Positive	B False Positive
Test (–)	C False Negative	D True Negative

• Sensitivity = A / (A + C)
• Specificity = D / (D + B)
• Positive Predictive Value = A / (A + B)
• Negative Predictive Value = D / (C + D)
• Positive Likelihood Ratio = sensitivity / (1 – specificity) 
• Negative Likelihood Ratio = (1 – sensitivity) / specificity 
• Number needed to treat (NNT) = 1 / (absolute risk reduction)