Renal Nutrition Specialist (CSR) Exam

The kidneys perform essential functions including filtration of metabolic waste products, regulation of fluid and electrolyte balance, maintenance of acid-base homeostasis, blood pressure regulation, erythropoietin production, and activation of vitamin D. Renal disease occurs when these functions become impaired due to acute injury or chronic progressive damage.

Renal diseases are classified as acute kidney injury (AKI) or chronic kidney disease (CKD). AKI represents a sudden decline in kidney function occurring over hours to days, characterized by rapid increases in serum creatinine and blood urea nitrogen with potential oliguria or anuria. CKD represents progressive, irreversible loss of kidney function over months to years.

Primary causes of renal disease include diabetes mellitus (leading cause accounting for approximately 44% of new ESRD cases), hypertension (second leading cause at approximately 28%), glomerulonephritis, polycystic kidney disease, interstitial nephritis, obstructive uropathy, and nephrotoxic drug exposure. Secondary causes include systemic lupus erythematosus, amyloidosis, multiple myeloma, and hepatorenal syndrome.

Risk factors for renal disease progression include uncontrolled diabetes, poorly managed hypertension, proteinuria, obesity, smoking, cardiovascular disease, family history of kidney disease, African American or Hispanic ethnicity, and age over 60 years.

Clinical manifestations depend on disease stage and include edema, hypertension, proteinuria, hematuria, decreased urine output, fatigue, nausea, anorexia, pruritus, and cognitive changes in advanced stages. Uremic symptoms appear when GFR falls below 15 mL/min/1.73 m² and include nausea, vomiting, metallic taste, hiccups, pericarditis, encephalopathy, and peripheral neuropathy.

CKD is defined as kidney damage or GFR below 60 mL/min/1.73 m² persisting for three months or longer. The KDIGO classification system stages CKD based on GFR and albuminuria categories.

GFR Categories:

Albuminuria Categories:

CKD progression involves nephron loss leading to compensatory hyperfiltration in remaining nephrons, which accelerates further damage. Proteinuria indicates glomerular damage and predicts faster progression. Each doubling of proteinuria increases CKD progression risk by approximately 30%.

Nutrition implications vary by CKD stage. Early stages (G1-G2) focus on managing underlying conditions and cardiovascular risk reduction. Moderate stages (G3a-G3b) require attention to phosphorus, potassium, and protein intake. Advanced stages (G4-G5) necessitate comprehensive dietary modifications including protein restriction, strict phosphorus and potassium control, sodium limitation, and fluid management.

Protein recommendations for CKD stages 3-5 not on dialysis range from 0.55-0.60 g/kg/day for metabolically stable patients without diabetes, or 0.6-0.8 g/kg/day for patients with diabetes or those at risk for protein-energy wasting. Higher protein intake (1.0-1.2 g/kg/day) is recommended for patients on maintenance hemodialysis or peritoneal dialysis to compensate for dialytic losses.

Blood urea nitrogen (BUN) represents the nitrogen content of urea, a metabolic waste product of protein catabolism. Urea is synthesized in the liver from ammonia generated during amino acid deamination, then filtered by the glomeruli and partially reabsorbed in the tubules.

Normal BUN ranges from 7-20 mg/dL in adults. Elevated BUN (azotemia) occurs with decreased GFR, increased protein catabolism, gastrointestinal bleeding, dehydration, high protein intake, corticosteroid therapy, or catabolic states such as sepsis and trauma.

The BUN-to-creatinine ratio helps differentiate prerenal azotemia from intrinsic renal disease. Normal ratio is 10:1 to 20:1. Ratios greater than 20:1 suggest prerenal causes (dehydration, heart failure, gastrointestinal bleeding, high protein intake, catabolic states). Ratios less than 10:1 suggest liver disease, low protein intake, or rhabdomyolysis.

BUN has limitations as a kidney function marker because it is affected by non-renal factors including dietary protein intake, gastrointestinal bleeding, hydration status, and catabolic rate. BUN may remain within normal limits despite significant GFR reduction if protein intake is restricted. Conversely, BUN may be elevated with normal kidney function during high-protein feeding or upper gastrointestinal bleeding.

In dialysis patients, BUN is used to calculate urea kinetics and dialysis adequacy. Pre-dialysis BUN levels typically range from 60-100 mg/dL in hemodialysis patients. Post-dialysis BUN should be at least 65-70% lower than pre-dialysis values for adequate dialysis.

The urea reduction ratio (URR) is calculated as: URR = (Pre-dialysis BUN - Post-dialysis BUN) / Pre-dialysis BUN × 100. Target URR is ≥65% for adequate hemodialysis.

Glomerular filtration rate represents the volume of plasma filtered by the glomeruli per unit time and is the best overall measure of kidney function. Normal GFR is approximately 120-130 mL/min/1.73 m² in young adults and declines with age by approximately 1 mL/min/1.73 m² per year after age 30.

Direct GFR measurement using inulin clearance is impractical for routine clinical use. Estimated GFR (eGFR) is calculated using creatinine-based or cystatin C-based equations.

The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation is the preferred method for eGFR calculation. The 2021 CKD-EPI equation eliminates the race coefficient and uses creatinine and/or cystatin C:

eGFR (creatinine) = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^-1.200 × 0.9938^age × 1.012 (if female)

Where Scr is serum creatinine, κ is 0.7 for females and 0.9 for males, α is -0.241 for females and -0.302 for males.

Factors affecting serum creatinine and eGFR accuracy include muscle mass (lower in elderly, amputees, and those with muscle wasting), dietary protein and creatine intake, medications affecting tubular secretion (trimethoprim, cimetidine), and analytical interferences.

eGFR should be interpreted cautiously in extremes of body size, muscle mass, or dietary intake. Cystatin C-based equations may be more accurate in these populations as cystatin C production is less dependent on muscle mass.

Clinical applications of eGFR include CKD staging, medication dosing adjustments, timing of dialysis initiation planning, transplant evaluation, and monitoring disease progression. Serial eGFR measurements allow calculation of GFR slope to predict time to kidney failure.

Metabolic acidosis in CKD results from decreased renal capacity to excrete hydrogen ions and regenerate bicarbonate. As GFR declines below 30 mL/min/1.73 m², the kidneys cannot excrete the daily acid load from protein metabolism (approximately 1 mEq/kg/day).

Normal serum bicarbonate ranges from 22-28 mEq/L. Metabolic acidosis is defined as serum bicarbonate below 22 mEq/L. In CKD, acidosis is typically high anion gap type due to retention of sulfates, phosphates, and organic anions.

Anion gap is calculated as: Anion Gap = Na - (Cl + HCO3). Normal anion gap is 8-12 mEq/L. Elevated anion gap in CKD results from accumulation of unmeasured anions normally excreted by the kidneys.

Consequences of chronic metabolic acidosis include muscle protein catabolism leading to muscle wasting, bone mineral dissolution contributing to renal osteodystrophy, growth retardation in children, increased inflammation, insulin resistance, accelerated CKD progression, and increased mortality risk.

Treatment targets serum bicarbonate at 22-26 mEq/L. Oral alkali therapy using sodium bicarbonate (650-1300 mg twice daily initially, titrated to effect) or sodium citrate is first-line treatment. Dietary modifications include emphasizing fruits and vegetables (which provide base-forming potential) while limiting animal protein (which provides acid load).

In dialysis patients, dialysate bicarbonate concentration (typically 35-40 mEq/L in hemodialysis, 35-40 mEq/L in peritoneal dialysis) provides bicarbonate replacement. Pre-dialysis bicarbonate levels should be maintained at 22-24 mEq/L.

Phosphorus homeostasis depends on dietary intake, intestinal absorption, bone resorption/formation, and renal excretion. Approximately 80-90% of filtered phosphorus is reabsorbed by the proximal tubule. As GFR declines, phosphorus excretion decreases, leading to hyperphosphatemia.

Normal serum phosphorus ranges from 2.5-4.5 mg/dL in adults. In CKD stages 3-5, phosphorus levels often remain normal until GFR falls below 30 mL/min/1.73 m² due to compensatory increases in PTH and FGF-23, which enhance phosphorus excretion per remaining nephron.

Hyperphosphatemia consequences include secondary hyperparathyroidism, renal osteodystrophy, soft tissue and vascular calcification, increased cardiovascular mortality, and CKD progression. The calcium-phosphorus product (Ca × P) should be maintained below 55 mg²/dL² to reduce calcification risk.

KDIGO guidelines recommend maintaining serum phosphorus toward the normal range in CKD stages 3-5 and lowering elevated phosphorus levels toward normal in dialysis patients. Specific targets are individualized based on trends and concurrent treatment of CKD-MBD.

Dietary phosphorus restriction is the foundation of management. Target intake is 800-1000 mg/day for CKD stages 3-5 and dialysis patients. Phosphorus sources include organic phosphorus (from protein-containing foods, approximately 40-60% absorbed) and inorganic phosphorus additives (from processed foods, up to 90-100% absorbed). Patients should be educated to read ingredient labels and avoid foods with phosphorus-containing additives.

Phosphate binders are prescribed when dietary restriction alone is insufficient. Calcium-based binders (calcium carbonate, calcium acetate) bind dietary phosphorus in the intestine, forming insolite calcium phosphate complexes. However, calcium-based binders contribute to hypercalcemia and vascular calcification risk. Non-calcium-based binders (sevelamer, lanthanum carbonate, ferric citrate, sucroferric oxyhydroxide) are preferred for patients with hypercalcemia, low PTH, or vascular calcification.