The kidneys constantly clean the blood of waste products and excess fluid through filtration. The efficiency of this function is measured by the Glomerular Filtration Rate (GFR). GFR represents the volume of blood plasma filtered by the kidneys per minute and is the most widely accepted indicator of overall kidney function. Tracking GFR is important in children because it allows doctors to detect kidney damage early, monitor the progression of underlying conditions, and safely adjust medication dosages.
Understanding Glomerular Filtration Rate
GFR reflects the work done by the nephrons, the microscopic filtering units within the kidney. Filtration begins in the glomerulus, a small cluster of capillaries within the nephron. This structure acts like a highly selective sieve, allowing small molecules like waste products and water to pass into the renal tubule.
The filtration membrane restricts the passage of larger elements, such as blood cells and most proteins, ensuring they remain in the bloodstream. GFR is the volume of filtrate formed by all the glomeruli combined each minute. This process is driven by hydrostatic pressure, the force of blood pushing against the capillary walls.
GFR is a dynamic measure that indicates how efficiently metabolic wastes are being removed from the body. A healthy GFR ensures that the body maintains a balance of water, electrolytes, and acid-base levels, which is crucial for overall homeostasis. If the GFR declines, it signals that the kidneys are losing filtering capacity, leading to a buildup of toxins in the blood.
Unique Challenges in Measuring Pediatric GFR
Calculating GFR in children is significantly more complex than in adults due to the physiological changes that occur during growth and maturation. Direct measurement using external markers (e.g., iohexol or inulin) requires intravenous injection and multiple blood draws. This makes it an impractical and invasive procedure for routine pediatric care. As a result, clinicians rely on estimated GFR (eGFR), calculated using mathematical formulas.
These formulas use blood biomarkers, primarily serum creatinine and cystatin C. Serum creatinine, a waste product of muscle metabolism, is an imperfect marker in children because its level relates directly to the child’s muscle mass. Since muscle mass changes rapidly during development, especially in infancy and puberty, creatinine levels can be low even if kidney function is mildly reduced.
Cystatin C, a protein produced by most cells, is less dependent on muscle mass and is considered a more stable marker in some pediatric populations. However, it is more expensive, not always readily available, and lacks standardized measurement methods.
To account for these variables, different pediatric-specific equations have been developed. These include the updated Bedside Schwartz formula, which incorporates the child’s height and creatinine, and the CKiD (Chronic Kidney Disease in Children) formula, which may include both creatinine and cystatin C. The choice of formula depends on the child’s age and the available lab results.
Interpreting Pediatric GFR Results
The resulting GFR number translates directly into a diagnosis of kidney function, but the “normal” value is highly dependent on age. A healthy newborn’s GFR is naturally lower, often between 40 and 60 mL/min/1.73 m\(^2\). This rate gradually rises to adult levels by about one to two years of age. For children over two years old, a GFR of 90 mL/min/1.73 m\(^2\) or higher is considered normal.
When GFR falls below the expected range, it is used to stage Chronic Kidney Disease (CKD) based on standardized clinical guidelines. For children over two, CKD staging is defined as follows:
- Stage 1: GFR of 90 or above, but with other signs of kidney damage (e.g., protein in the urine).
- Stage 2: Mild decrease in GFR, ranging from 60 to 89 mL/min/1.73 m\(^2\), with evidence of damage.
- Stage 3a: Mild-to-moderate reduction in function, GFR between 45 and 59 mL/min/1.73 m\(^2\).
- Stage 3b: Moderately-to-severely decreased function, GFR from 30 to 44 mL/min/1.73 m\(^2\).
- Stage 4: Severe GFR reduction, falling between 15 and 29 mL/min/1.73 m\(^2\).
- Stage 5: Kidney failure, with a GFR below 15 mL/min/1.73 m\(^2\).
For children under two, CKD staging is often based on how many standard deviations the GFR is below the mean for that specific age.
Next Steps Following an Abnormal GFR
If a child’s estimated GFR is confirmed to be low, the next step involves further investigation and careful monitoring. Initial testing includes a detailed urine analysis to check for protein or blood, along with imaging studies like an ultrasound to assess the kidney structure. The frequency of repeat GFR testing is determined by the severity of the low result, with more frequent monitoring recommended for those at higher risk of progression.
A diagnosis of moderately or severely decreased GFR warrants a referral to a pediatric nephrologist. Management strategies begin with controlling blood pressure, as hypertension can accelerate kidney damage. Dietary adjustments are also introduced, focusing on adequate protein and energy intake to promote healthy growth. Identifying and addressing underlying causes, such as recurrent urinary tract infections or congenital abnormalities of the urinary tract, is a primary goal in preserving remaining kidney function.