Assessing Kidney Function in Children: GFR Estimation, CKD Staging, and Electrolyte Interpretation

Summary

• Bedside Schwartz formula: GFR = 0.413 × height (cm) / serum creatinine (mg/dL); height-based because creatinine production tracks muscle mass, not weight [1].
• CKD stages by GFR (mL/min/1.73m²) are the same cutoffs used in adults: Stage 1 ≥90, Stage 2 60-89, Stage 3a 45-59, Stage 3b 30-44, Stage 4 15-29, Stage 5 <15.
• CKiD U25 equation is preferred over bedside Schwartz for tracking known CKD in patients aged 1-25; it uses creatinine, cystatin C, BUN, height, and sex, and the 2021 update removed the race coefficient [3,4].
• FENa <1% suggests prerenal AKI, 1-2% is indeterminate, >2% suggests intrinsic AKI; unreliable on diuretics or with pre-existing CKD [5].
• Urine anion gap: negative value indicates appropriate renal acid excretion (GI or extrarenal bicarbonate loss); positive value indicates failure of renal acid excretion (RTA, hypoaldosteronism) [6].
• TTKG <2 with hypokalemia points to extrarenal potassium loss; TTKG >4 with hyperkalemia indicates appropriate renal potassium secretion.

GFR estimation

Bedside Schwartz has known failure modes:

• Diet affects creatinine; high-meat intake raises it.
• Reduced muscle mass (e.g., cerebral palsy, malnutrition) produces falsely low creatinine for height, which causes Schwartz to overestimate GFR.
• In young infants, creatinine is low enough that ordinary lab variation swings the calculated GFR widely.

24-hour urine creatinine clearance is more accurate in these situations, but accurate collection in an outpatient 4-year-old is difficult. Inpatient collection is more reliable than home collection.

Clinical pearl. A GFR that drops from 80 to 65 over a year indicates progressive disease even if each individual value would be read as "normal" or "mild loss" in isolation. Trend the number, not just the stage.

Open the Schwartz GFR Calculator →

CKD staging

These cutoffs are unchanged from adult staging. For a child aged 1-25 with established CKD, use CKiD U25 rather than bedside Schwartz to assign stage and track progression, since it incorporates cystatin C and BUN in addition to creatinine and height [3,4].

Open the CKiD U25 Calculator →

Diagnostic urine indices

FENa distinguishes prerenal AKI (hypoperfusion, tubules intact and sodium-avid) from intrinsic AKI (tubular damage, sodium not reabsorbed) [5].

Caution. FENa is unreliable in patients on diuretics or with pre-existing CKD. Do not use it to classify AKI in a child already on furosemide or with known chronic kidney disease.

Urine anion gap answers whether the kidney is appropriately excreting acid in a normal anion gap metabolic acidosis [6].

TTKG answers whether the kidney is the source of a potassium disturbance.

Clinical pearl. A FENa that shifts from <1% toward >2% during resuscitation can indicate evolving intrinsic injury (acute tubular necrosis). A FENa that falls toward <1% with fluid resuscitation suggests improving renal perfusion. Repeat the calculation rather than relying on a single value.

A structured approach to a nephrology consult:

1. Start with GFR: bedside Schwartz for a quick screen, CKiD U25 if the patient has known CKD [1,3,4].
2. Check urine for protein, blood, and casts.
3. In AKI, calculate FENa to separate prerenal from intrinsic injury [5].
4. In normal anion gap metabolic acidosis, add urine anion gap [6].
5. In unexplained hypokalemia or hyperkalemia, add TTKG.

Open the FENa Calculator → Open the Urine Anion Gap Calculator → Open the Urine K/Cr Ratio Calculator →

Special populations

• Infants. Serum creatinine starts around 0.3 mg/dL or lower and rises toward adult values by the teenage years. Reference ranges must be age- and sex-specific.
• Premature infants. Creatinine is lower for chronologic age than in term infants. Use post-conceptional age, not chronologic age, when estimating GFR, or GFR will be overestimated.
• Reduced muscle mass. Cerebral palsy or malnutrition lowers creatinine independent of true kidney function, causing bedside Schwartz to overestimate GFR. Interpret the number against the full clinical picture, not in isolation.

Caution. In a child with reduced muscle mass, a "normal" Schwartz-estimated GFR does not rule out significant kidney disease. Consider creatinine clearance or cystatin C-based estimation (as used in CKiD U25) when muscle mass is atypical [3,4].

References

1. Schwartz GJ, Muñoz A, Schneider MF, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol. 2009;20(3):629-637. doi:10.1681/ASN.2008030287
2. Counahan R, Chantler C, Ghazali S, Kirkwood B, Rose F, Barratt TM. Estimation of glomerular filtration rate from plasma creatinine concentration in children. Arch Dis Child. 1976;51(11):875-878. doi:10.1136/adc.51.11.875
3. Pierce CB, et al. Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults. Kidney Int. 2021;99(4):948-956. doi:10.1016/j.kint.2020.10.047
4. Pierce CB, Muñoz A, Engber TM, et al. Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease. J Am Soc Nephrol. 2021;32(11):2696-2712. doi:10.1016/j.kint.2020.10.047
5. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581. doi:10.1001/jama.1976.03270060029022
6. Batlle DC, Hizon M, Cohen E, Gutterman C, Gupta R. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med. 1988;318(10):594-599. doi:10.1056/NEJM198803103181002