Renal Function Tests

**Renal Function Tests** **Glomerular Filtration Rate (GFR)** * Normal GFR = 90-120 mL/min/1.73m² * Varies considerably between males and females. * Normal GFR is 120 mL/min/1.73m² in young adults. * Progressive decline in GFR with age. **Methods of estimating GFR:** * **Cockcroft-Gault (CG) equation:** * Used for estimating GFR in adults. * Not accurate in all cases, particularly in older adults. * **Modification of Diet in Renal Disease (MDRD) Study Equation:** * More accurate than the CG equation. * Includes additional factors like age, sex, and race. * **Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation:** * Most accurate equation for estimating GFR. * Takes into account factors like age, sex, race, and serum creatinine. **Serum Creatinine:** * Indirect indicator of renal function. * Less sensitive to changes in GFR, especially in early stages of kidney disease. * Can be affected by factors like diet, muscle mass, and exercise. * Not a good assessor of renal function in individuals with low muscle mass. **Interfering factors with these tests:** * Low GFR levels * Low muscle mass * Pregnancy * Exercise * Diet * Certain medications **CREATININE CLEARANCE** * **S. Creatinine:** * Overestimates GFR due to minimal Cr secretions in proximal tubules. * Precise evaluation of Cr clearance needs Cr measurement over a 24-hour period. * Calculation of Cr clearance: Cr clearance = (Ucr x V) / Pcr where: Ucr = urine creatinine concentration V = urine volume in ml/min (collecting urine for 24 hours, the unit is usually ml/day, has to be converted into ml/min) Pcr = plasma creatinine concentration **ESTIMATED GFR (eGFR)** * Calculated using [Scr] & demographic data. * Several Equations can be used: **Cockcroft-Gault Equation:** ``` CrCl = (140-age) x weight (kg) x (0.85 if female) / (72 x Scr) ``` **Modification of Diet in Renal Disease (MDRD) Study Equation:** ``` GFR = 186 x (Scr)^-1.154 x (age)^-0.203 x (0.742 if female) x (1.210 if African American) ``` **Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) Equation:** ``` GFR = 141 x min(Scr/κ, 1)^α x max(Scr/κ, 1)^-1.209 x 0.993^age x (1.018 if female) x (1.159 if African American) ``` where: κ = 0.7 for females and 0.9 for males **SERUM CYSTATIN C** * More precise indicator of GFR than Scr. * Small protein that cysteine proteases break down, made by ALL NUCLEATED CELLS. * More complex & expensive. * Lower "blind range" than Scr. **Blood Urea Nitrogen (BUN)** * Waste product produced by the liver in the urea cycle after protein digestion. * Filtered & excreted by the kidneys. * Elevated with: * High protein diet * Kidney disease * Increased protein catabolism * Can help diagnose underlying cause of AKI **Kidney Disease Improving Global Outcomes (KDIGO) criteria for staging of AKI:** **Stage** | **Serum Creatinine** | **Urine Output** ---|---|---| AKI Stage 1 | ≥1.5 x baseline | <0.5 mL/kg/hr for 6-12 hours AKI Stage 2 | ≥2.0 x baseline | <0.5 mL/kg/hr for ≥12 hours AKI Stage 3 | ≥3.0 x baseline | Anuria for ≥12 hours OR SCr ≥5 mg/dL with a known baseline SCr ≥4.0 mg/dL *If serum creatinine and urine output criteria correspond to different stages, consider staging based on the criterion that corresponds to the highest stage.* **FRACTIONAL EXCRETION OF Na (FENa)** * **Definition:** % of GFR (Na filtered) that is excreted into the urine. * **Usage:** * Distinguishing between pre-renal & intrarenal AKI * Helps distinguish between renal vs. extrarenal etiology in hypotonic hyponatremia * **Calculations:** ``` FENa = (UNa x Pcr) / (PNa x Ucr) ``` where: UNa = Urine sodium Pcr = Plasma creatinine PNa = Plasma sodium Ucr = Urine creatinine