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

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

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

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 

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). 

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) 

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 

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

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)] 

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) 

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.  

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**  

SVR = [(MAP – CVP) / CO] x 80  

{normal 700–1600 dynes*s/cm5} 

PVR = [(MPAP – PCWP) / CO] x 80  

{normal 20–120 dynes*s/cm5} 

Pulse Pressure = SBP – DBP 

MAP = [SBP + (DBP x 2)] / 3 

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 

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)