MDCalc » Renal

Sodium Correction Rate in Hyponatremia

admin — Mon, 05 Oct 2009 22:34:19 +0000

Change in Serum Sodium = (Fluid Sodium - Serum Sodium) / (Total Body Water + 1)

Total Body Water = (Wt in kg * % Water)

Age	Male	Female
Child	0.6	0.6
Adult	0.6	0.5
Elderly	0.5	0.45

*Experts recommend correcting no faster than 12 mmol/L/day (0.5mmol/L/hr) to avoid central pontine myelinolysis (first calculation), and only correcting it faster — and only using hypertonic (3%) saline — if the patient is seriously symptomatic at a rate of 1-2 mmol/L/hr (second/third calculations), and even then, only correcting it at 1-2 mmol/L/hr for <3 hours.

Hyponatremia. Adrogué HJ, Madias NE. NEJM, 2000.

Free Water Deficit in Hypernatremia

admin — Fri, 02 Jan 2009 04:34:27 +0000

Free Water Deficit = Total Body Water * ((Curent Serum Na/Desired Serum Na)-1)

Total Body Water = (Wt in kg * % Water)

Age	Male	Female
Child	0.6	0.6
Adult	0.6	0.5
Elderly	0.5	0.45

Transtubular Potassium Gradient (TTKG)

admin — Fri, 02 Jan 2009 04:33:15 +0000

Transtubular Potassium Gradient (TTKG) = (P_Osm * U_Potassium ) / (P_Potassium x U_Osm)

Note that this formula is valid only when Uosm >300 and UNa >25. This test can help determine the source of potassium imbalance.

Sodium Deficit in Hyponatremia

admin — Fri, 02 Jan 2009 04:28:13 +0000

Sodium Deficit = Total Body Water * Normal Wt in kg * (Desired Na - Pt's Na) (TBW = 0.6 if male and 0.5 if female)

Remember not to correct too rapidly to avoid central pontine myelinolysis!

Serum Osmolality/Osmolarity

admin — Fri, 02 Jan 2009 04:26:11 +0000

Serum Osmolality (US) = (2 * (Na) + (BUN / 2.8) + (glucose / 18) + (ethanol/4.6); SI units ares all molar, so no need to divide by 2.8 or 18 or 4.6.

Pediatric Ins and Outs (per kg and per hour)

admin — Fri, 02 Jan 2009 04:24:02 +0000

Just, uh, divide.

Often on inpatient pediatrics services, ins are recorded as mL/kg and outs are recorded as mL/kg/hour.

MDRD GFR Equation

admin — Fri, 02 Jan 2009 04:17:37 +0000

GFR = 186.3 x SerumCr^-1.154 * age^-0.203 * 1.212 (if patient is black) * 0.742 (if female)

Levey AS, Greene T, Kusek JW, et al. A simplified equation to predict glomerular filtration rate from serum creatinine (Abstr) J Am Soc Nephrol 2000;(11):155A

Maintenance Fluids Calculations

admin — Fri, 02 Jan 2009 03:56:41 +0000

Calculated Using the "4-2-1" Rule:

For 0-10kg: 4 mL/kg/hr
For 10-20kg: + 2 mL/kg/hr
For >20kg: + 1 mL/kg/hr

Fractional Excretion of Urea (FEUrea)

admin — Fri, 02 Jan 2009 03:51:58 +0000

Fractional Excretion of Urea (FEUrea) = (Serum_Cr * U_Urea ) / (Serum_Urea x U_Cr) %

This test can provide similar information to the FENa equation, but can still be used in patients on diuretic therapy (diuretics alter the sodium concentration, making the FENa equation unusable).

Fractional Excretion of Sodium (FENa)

admin — Fri, 02 Jan 2009 03:51:19 +0000

Fractional Excretion of Sodium (FENa) = (P_Cr * U_Na ) / (P_Na x U_Cr) %

	Prerenal	Intrinsic Renal	Postrenal
U_Na (mmol/L)	<20	>40	>40
FE_Na	<1%	>1%	>4%

Prerenal: Anything that causes decreased effective renal perfusion: Hypovolemia, CHF, Renal Artery Stenosis, Sepsis, etc. Remember, contrast-induced nephropathy will often look pre-renal.

Intrinsic Renal: ATN, AIN, Glomerulonephritides, etc

Postrenal: Obstruction (BPH, bladder stone, bilateral ureter obstruction)

Is patient female?	Yes
Age	Child Adult Elderly
Normal Pt Weight	lbs
Serum Na	mmol/L
Type of Fluid	3% Saline (513 Na mmol/L) Normal Saline (154 Na mmol/L) Ringer's Lactate (130 Na mmol/L) 1/2 Normal Saline (77 Na mmol/L)
Fluid Rate to Increase Sodium by 0.5 mmol/L/hr	ml/hr of selected fluid
Fluid Rate to Increase Sodium by 1 mmol/L/hr*	ml/hr of selected fluid
Fluid Rate to Increase Sodium by 2 mmol/L/hr*	ml/hr of selected fluid

Is patient female?	Yes
Age	Child Adult Elderly
Normal Pt Weight	lbs
Serum Na	mEq/dL
Desired Na	mEq/dL
Water Deficit	L

Serum Osm	mOsm/L
Urine Osm	mOsm/L
Serum K	mg/dL
Urine K	mg/dL
TTKG

Is patient female?	Yes
Normal Pt Weight	lbs
Serum Na	mEq/L
Desired Na	mEq/L
Sodium Deficit	mEq

Na	mEq/L
BUN	mg/dL
Glucose	mg/dL
Ethanol	mg/dL
Serum Osmolality	mOsm/kg
Normal serum osmolality = 285-295 mOsm/kg Normal Serum Osm Gap (Measured-Calculated) is -14 to +10.

Volume to Be Calculated	mL
Weight	kg
Over How Many Hours?	hours
Volume per Weight	mL/kg
Volume per Weight per Hour	mL/kg/hour

Only for chronic kidney disease (CKD), not accurate for acute renal failure. Please also note that a recent study indicates the MDRD may underestimate the actual GFR in healthy patients by up to 29%.
Age	years old
Serum Creatinine	mg/dL
Patient is female.	Yes
Patient is black.	Yes
GFR	ml/min/1.73 m²

Weight	lbs
Maintenance Fluids	mL/hr
1.5 x Maintenance Fluids (Peds)	mL/hr
Pediatric Fluid Bolus	mL
A pediatric bolus is generally calculated as 20mL/kg.

Serum BUN	mEq/L
Urine Urea	mEq/L
Serum Creatinine	mg/dL
Urine Creatinine	mg/dL
FEUrea	%