Assessing kidney function in kids is trickier than in adults, their GFR changes with age, creatinine means something different at 6 months than at 16 years, and you can miss disease if you're not careful with reference ranges. Get this wrong and you miss early CKD or underestimate drug toxicity.
Estimating GFR: The Schwartz Formula
GFR = 0.413 × (height in cm) / serum creatinine
This is your bedside workhorse. Height matters in kids because creatinine production tracks with height (and muscle mass), not weight. A 5-foot 12-year-old doesn't produce creatinine like a 5-foot 8-year-old.
The formula has weaknesses. Diet affects creatinine (high-meat diets raise it). Muscle mass varies: a kid with cerebral palsy and atrophic muscles will have falsely low creatinine for their height. Young infants are tough because their creatinine is so low that small lab variation swings the GFR wildly.
Use the Schwartz GFR calculator to stay consistent, especially across your team.
24-hour urine creatinine clearance is more accurate than Schwartz when GFR is already low, when body habitus is unusual, or when you're not sure about muscle mass. The math is straightforward:
Clearance = (Urine creatinine × Urine volume) / (Plasma creatinine × Time)
The catch is actually collecting urine. Getting accurate 24-hour collection from a 4-year-old or outpatient is difficult. In hospital it's doable; at home, it's challenging.
Use the Creatinine Clearance calculator once you have the numbers.
How We Stage CKD
The five stages are the same for kids and adults:
- Stage 1: GFR ≥90 (kidney damage but preserved function)
- Stage 2: GFR 60–89 (mild loss)
- Stage 3a: GFR 45–59 (moderate loss)
- Stage 3b: GFR 30–44 (more moderate)
- Stage 4: GFR 15–29 (severe)
- Stage 5: GFR <15 (failure)
For children and young adults (aged 1–25) with established CKD, the CKiD U25 equation provides more accurate GFR estimation than bedside Schwartz. It incorporates serum creatinine, cystatin C, BUN, height, and sex (the 2021 updated equation removed the race coefficient to reduce bias).
Use CKiD U25 when you're tracking progression in a kid with known disease.
When You Need to Know What the Kidneys Are Actually Doing
Fractional Excretion of Sodium (FENa)
The math: FENa = [(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100%
This one question: is the kid's AKI from bad perfusion (prerenal) or intrinsic kidney damage? In prerenal AKI, the tubules are intact and holding onto sodium. In intrinsic damage, they're not.
- FENa <1%: Tubules working, looks like prerenal (dehydration, shock)
- FENa 1–2%: Gray zone, depends on context
- FENa >2%: Tubules broken, intrinsic kidney disease
Caveat: diuretics or existing CKD confound this. Don't rely on it if the kid's already on lasix or has chronic kidney disease.
Use FENa calculator to do the math.
Urine Anion Gap
For normal anion gap metabolic acidosis, you need to know: is the kidney responding appropriately by excreting acid (making the urine acidic) or not?
UAG = (Urine Na + Urine K) − Urine Cl
- Negative UAG: Kidney is doing its job, acid is being lost through GI or there's extra bicarbonate loss
- Positive UAG: Kidney isn't excreting acid (RTA, hypoaldosteronism, or sick ammoniagenesis)
Catches renal tubular acidosis fast.
Use Urine Anion Gap calculator.
Transtubular Potassium Gradient (TTKG)
When a kid has hypokalemia or hyperkalemia and you want to know if it's the kidney's fault:
TTKG = (Urine K × Serum osmolality) / (Serum K × Urine osmolality)
- TTKG <2 in hypokalemia: Kidney is conserving potassium (loss is GI, not renal)
- TTKG >4 in hyperkalemia: Kidney is secreting potassium (appropriate response)
- Low TTKG in hyperkalemia: Red flag, tubular dysfunction or aldosterone problem, kidney can't clear it
Use TTKG calculator.
Pulling It All Together
Don't order one test and declare victory. Start with GFR (Schwartz for fast screening, CKiD U25 if you're tracking known CKD), check the urine (protein, blood, casts), and if there's AKI calculate FENa to distinguish prerenal from intrinsic. In metabolic acidosis, throw in UAG. If electrolytes are weird, grab TTKG.
Single values lie. Trends tell the story. A GFR that drops from 80 to 65 over a year means progressive disease. FENa that transitions from <1% (prerenal) to >2% during resuscitation may indicate evolving intrinsic injury (acute tubular necrosis), while a FENa that decreases toward <1% with fluid resuscitation suggests improving renal perfusion.
Age and Body Matters
Creatinine is age and sex dependent. Infants start around 0.3 mg/dL or lower; teenage boys approach adult values. Premature infants are special, their creatinine is lower for their chronologic age, so plug in post-conceptional age or you'll overestimate GFR.
Conditions that reduce muscle mass (cerebral palsy, malnutrition) lower creatinine independent of kidney function. This means Schwartz will overestimate GFR. You need the full clinical picture.
Know Schwartz for quick bedside estimates. Use CKiD U25 when you're tracking progression. Calculate FENa in AKI to figure out what's broken. Use UAG and TTKG to solve electrolyte and acid-base puzzles. Look at trends, not single numbers. And remember that creatinine doesn't mean the same thing in a premature infant or a kid with muscle disease.