Renal Nutrition Specialist (CSR) Exam

Hemodialysis removes waste products and excess fluid from the blood using an extracorporeal circuit and semipermeable membrane (dialyzer). Blood is pumped from the patient's vascular access through the dialyzer, where it is exposed to dialysate flowing in the opposite direction. Solutes move across the membrane by diffusion (concentration gradient) and convection (solute drag with water removal). Fluid is removed by ultrafiltration (pressure gradient).

Standard hemodialysis is performed three times weekly for 3-4 hours per session. Longer and more frequent dialysis sessions improve outcomes, leading to alternative schedules including short daily dialysis (2-3 hours, 5-6 days/week) and nocturnal hemodialysis (6-8 hours, 3-6 nights/week).

Vascular access options include arteriovenous fistula (AVF), arteriovenous graft (AVG), and central venous catheter (CVC). Native AVF is preferred due to lower infection and thrombosis rates and longer patency. AVF requires 2-4 months maturation before use. AVG provides earlier use (2-4 weeks) but has higher complication rates. CVC should be avoided when possible due to highest infection rates, stenosis risk, and mortality.

Dialysis adequacy is measured using Kt/V and urea reduction ratio. Kt/V represents the product of dialyzer clearance (K), time (t), and distribution volume (V). Target single-pool Kt/V is ≥1.4 per session for thrice-weekly hemodialysis. Kt/V below 1.2 is associated with increased mortality.

Nutrition considerations for hemodialysis patients include protein requirements of 1.0-1.2 g/kg/day to offset dialytic amino acid losses (approximately 10-12 g per session). Energy needs are 25-35 kcal/kg/day depending on age and activity level. Sodium restriction (2000-2300 mg/day) controls thirst and interdialytic weight gain. Potassium restriction (2000-3000 mg/day) prevents hyperkalemia. Phosphorus restriction (800-1000 mg/day) controls hyperphosphatemia. Fluid restriction limits interdialytic weight gain to 2-3 kg (or 3-5% of dry weight).

Intradialytic parenteral nutrition (IDPN) provides amino acids, dextrose, and lipids intravenously during hemodialysis for patients unable to meet needs orally. IDPN is indicated when oral or enteral intake is inadequate despite optimization and protein-energy wasting is present.

Peritoneal dialysis uses the peritoneal membrane as the dialyzer. Dialysate is instilled into the peritoneal cavity through a surgically placed catheter. Waste products and fluid are removed through diffusion and osmosis across the peritoneal membrane, then drained.

The peritoneal membrane consists of mesothelial cells, interstitium, and capillary endothelium. Membrane transport characteristics vary among patients and change over time. High transporters have rapid solute equilibration but poor ultrafiltration due to fast glucose absorption. Low transporters have slow solute clearance but excellent ultrafiltration.

Peritoneal dialysis modalities include continuous ambulatory peritoneal dialysis (CAPD), automated peritoneal dialysis (APD), and continuous cycling peritoneal dialysis (CCPD). CAPD involves manual exchanges performed 4-5 times daily with 2-3 L dwell volumes. APD uses a cycler machine to perform multiple exchanges overnight.

Dialysate solutions contain dextrose (1.5%, 2.5%, or 4.25%) as the osmotic agent, electrolytes, lactate or bicarbonate buffer, and calcium. Higher dextrose concentrations provide greater ultrafiltration but contribute to glucose absorption, hyperglycemia, weight gain, and potential membrane damage. Icodextrin, a glucose polymer, provides sustained ultrafiltration for long dwells without significant systemic absorption.

Peritoneal dialysis adequacy is measured using weekly Kt/V urea (target ≥1.7) and weekly creatinine clearance. Residual kidney function contributes significantly to overall clearance and should be preserved as long as possible.

Nutrition considerations for peritoneal dialysis include higher protein requirements (1.2-1.5 g/kg/day) due to peritoneal protein losses (5-15 g/day). Energy intake calculations must account for glucose absorption from dialysate (60-80% of instilled dextrose is absorbed), which provides approximately 300-800 kcal/day depending on exchange volume and dextrose concentration. Sodium and fluid restrictions may be more liberal than hemodialysis if adequate ultrafiltration is achieved. Potassium restriction is often unnecessary due to continuous dialysis and potassium losses into dialysate. Phosphorus restriction remains necessary.

Continuous ambulatory peritoneal dialysis (CAPD) is a manual technique performed without machines. The patient performs 4-5 exchanges daily, typically at waking, midday, evening, and bedtime. Each exchange involves draining spent dialysate (15-20 minutes), instilling fresh dialysate (10 minutes), and allowing the solution to dwell (4-8 hours for daytime exchanges, 8-10 hours overnight).

Standard CAPD prescription includes 2.0-2.5 L exchanges (adjusted for body size and tolerance), glucose concentrations selected based on ultrafiltration requirements, and dwell times optimized for membrane transport characteristics.

CAPD advantages include simplicity (no machine required), portability, continuous clearance maintaining stable biochemistry, preservation of residual kidney function, flexibility in daily activities, and lower cost. Disadvantages include time commitment (30-40 minutes per exchange, 4-5 times daily), potential body image concerns, abdominal distension affecting appetite, and infection risk with each connection.

Peritonitis is the most serious complication, presenting with cloudy effluent, abdominal pain, and fever. Exit-site and tunnel infections increase peritonitis risk. Proper aseptic technique during exchanges is essential for prevention.

Protein-energy wasting is common in CAPD patients due to continuous amino acid and protein losses into dialysate, glucose absorption suppressing appetite, and chronic inflammation. Nutritional monitoring should include serial assessments of dietary intake, anthropometrics, serum albumin, and prealbumin.

The peritoneal equilibration test (PET) characterizes peritoneal membrane transport characteristics to guide dialysis prescription. Standard PET uses 2 L of 2.5% dextrose dialysate with 4-hour dwell time. Dialysate and serum samples are collected at specified intervals.

PET results classify membrane transport as high, high-average, low-average, or low based on dialysate-to-plasma (D/P) creatinine ratio and dialysate glucose concentration at 4 hours (D/D0 glucose).

Modified PET (fast PET) uses abbreviated sampling and provides similar classification with less patient burden.

Membrane characteristics change over time. Long-term peritoneal dialysis may cause membrane thickening, neovascularization, and increased transport (high transporter characteristics). Encapsulating peritoneal sclerosis is a rare but serious complication involving membrane fibrosis and bowel obstruction.

Hyperphosphatemia management in dialysis patients requires a multimodal approach including dietary phosphorus restriction, phosphate binders, and adequate dialysis.

Dietary phosphorus control focuses on limiting high-phosphorus foods while maintaining adequate protein intake. Phosphorus-to-protein ratio guides food selection; foods with lower ratios provide protein with less phosphorus burden. Patients should avoid processed foods containing phosphorus additives (sodium phosphate, phosphoric acid, pyrophosphates) which are highly bioavailable. Cooking methods can reduce phosphorus content; boiling removes 30-50% of phosphorus from meats and vegetables.

Phosphate binders are categorized as calcium-based and non-calcium-based. Calcium-based binders include calcium carbonate (40% elemental calcium, 500-1500 mg with meals) and calcium acetate (25% elemental calcium, 667 mg tablets, 2-4 with meals). These are effective and inexpensive but contribute to positive calcium balance, hypercalcemia risk, and vascular calcification. Total elemental calcium from binders should not exceed 1500 mg/day.

Non-calcium-based binders include sevelamer (hydrochloride or carbonate, 800-1600 mg with meals), lanthanum carbonate (500-1000 mg chewed with meals), ferric citrate (210 mg ferric iron tablets, 2 with meals), and sucroferric oxyhydroxide (500 mg chewable tablets with meals). These avoid calcium loading and may reduce vascular calcification progression. Sevelamer also lowers LDL cholesterol. Ferric citrate and sucroferric oxyhydroxide provide iron supplementation but may cause dark stools.

Dialytic phosphorus removal averages 800-1000 mg per hemodialysis session (standard prescription) and 300-400 mg per day with peritoneal dialysis. Longer or more frequent dialysis improves phosphorus control.