CKD-MBD, Electrolytes, and Acid-Base Balance: Key Concepts for Healthcare

Normal Sodium (Na)

135-145 mEq/L

Normal Potassium (K)

3.5-5.0 mEq/L

Normal Chloride (Cl)

96-106 mEq/L

Normal Glucose

70-100 mg/dL

Normal CO2 (HCO3)

22-29 mEq/L

Normal Magnesium (Mg)

1.7-2.3 mg/dL

Normal Phosphorus (Phos)

2.5-4.5 mg/dL

Normal Calcium (Ca)

8.5-10.5 mg/dL

Normal pH

7.35-7.45

Normal pCO2

35-45 mmHg

Normal HCO3

22-26 mEq/L

Corrected Calcium Formula

Measured Ca + [0.8 × (4 - albumin)]

BUN:Scr Ratio >20:1

Usually indicates dehydration or volume depletion

Anion Gap Formula

Na - (Cl + HCO3)

ABG Step 1

Determine if pH indicates acidosis or alkalosis

ABG Step 2

Determine if disorder is respiratory (pCO2) or metabolic (HCO3)

ABG Step 3

Determine if compensation is occurring

Acidosis

pH <7.35

Alkalosis

pH >7.45

Respiratory Acidosis

Low pH with elevated pCO2

Respiratory Alkalosis

High pH with low pCO2

Metabolic Acidosis

Low pH with low HCO3

Metabolic Alkalosis

High pH with elevated HCO3

Compensation in Respiratory Acidosis

Kidneys retain HCO3 causing elevated bicarbonate

Compensation in Metabolic Acidosis

Lungs blow off CO2 causing decreased pCO2

COPD ABG Example

pH 7.32, pCO2 62, HCO3 31 = Chronic respiratory acidosis with renal compensation

CKD Effect on Vitamin D

Decreased renal activation of vitamin D

CKD Effect on Phosphorus

Increased phosphorus due to decreased excretion

CKD Effect on Calcium

Decreased calcium due to reduced vitamin D activation

CKD Effect on PTH

Increased PTH causing secondary hyperparathyroidism

Classic CKD-MBD Pattern

↑ Phosphorus, ↓ Calcium, ↑ PTH, ↓ Vitamin D

Secondary Hyperparathyroidism in CKD

Caused by low calcium, low vitamin D, and high phosphorus

PTH Effect on Bone

Increases bone resorption

Phosphate Binders Purpose

Reduce GI absorption of phosphorus

Examples of Phosphate Binders

Calcium acetate, sevelamer

Vitamin D Deficiency Treatment

Ergocalciferol (D2) or Cholecalciferol (D3)

When to Use Ergocalciferol or Cholecalciferol

Only when vitamin D level is low

Vitamin D Analog Drugs

Calcitriol, Paricalcitol, Doxercalciferol

Vitamin D Analog Effect on PTH

Decrease PTH

Vitamin D Analog Effect on Calcium

Increase calcium

Vitamin D Analog Effect on Phosphorus

Increase phosphorus

Calcitriol Effect on Calcium and Phosphorus

Increases both calcium and phosphorus

Calcitriol Use in CKD

Secondary hyperparathyroidism with normal vitamin D levels

Calcitriol Exam Favorite

Decreases PTH while increasing calcium and phosphorus

Cinacalcet Drug Class

Calcimimetic

Cinacalcet Brand Name

Sensipar

Cinacalcet Mechanism

Increases sensitivity of calcium-sensing receptor

Cinacalcet Effect on PTH

Decreases PTH

Cinacalcet Effect on Calcium

Decreases calcium

Cinacalcet Role in Therapy

Generally second-line for secondary hyperparathyroidism

Memory Aid for Cinacalcet

Sensipar makes receptor think calcium is high so PTH decreases

Vitamin D Deficiency Example

Low vitamin D → use ergocalciferol or cholecalciferol

Secondary Hyperparathyroidism Example

High PTH + normal vitamin D → use calcitriol

ESA

Erthyropoiesis-Stimulating Agent

Examples of ESA

Epoetin alfa, Darbepoetin alfa

ESA Purpose

Stimulate red blood cell production

ESA Black Box Warning

Increased risk of MI, stroke, VTE, vascular thrombosis, and death when Hb >11 g/dL

ESA Hemoglobin Goal

Avoid targeting Hb >11 g/dL

ESA Monitoring

Hemoglobin, ferritin, TSAT, blood pressure

ESA Additional Monitoring

Monitor for seizures during first few months

Acceptable Hemoglobin Increase on ESA

1-2 g/dL per month

ESA Mnemonic

ESA hates the number 11

TSAT Definition

Transferrin saturation

Iron Deficiency in CKD

Low serum iron and low TSAT indicate iron deficiency

Dialysis Patient Iron Rule

TSAT <30% and Ferritin <500 ng/mL indicates need for iron replacement

Preferred Iron in Dialysis Patients

IV iron

IV Iron Example

Iron sucrose

Ferritin Goal in Dialysis Patients

Maintain ferritin ≥500 ng/mL when evaluating iron therapy

TSAT Goal in Dialysis Patients

Maintain TSAT ≥30% when evaluating iron therapy

Anemia Case Example

Hgb 8 g/dL, TSAT 20%, Ferritin 355 ng/mL → Start IV iron sucrose

Do Not Increase ESA First

If iron deficiency is present, correct iron deficiency before escalating ESA

Hypervolemic Hyponatremia

Common in heart failure, cirrhosis, and fluid overload states

Signs of Hypervolemic Hyponatremia

Edema, JVD, crackles, low sodium

Treatment of Hypervolemic Hyponatremia

Sodium restriction and fluid restriction

Normal Saline in Hypervolemic Hyponatremia

Generally not appropriate

Hypovolemic Hypernatremia

Common with dehydration and fluid loss

Signs of Hypovolemic Hypernatremia

Dry mucous membranes, poor skin turgor, hypotension, tachycardia

Treatment of Hypovolemic Hypernatremia

Restore intravascular volume with 0.9% normal saline first

Hypertonic Saline (3%)

Main use is severe symptomatic hyponatremia

D5W + Loop Diuretic

Avoid in critically ill or hypovolemic patients

Potassium Replacement

Replace when K <3.5 mEq/L

Magnesium Replacement

Replace when Mg <1.7 mg/dL

Phosphorus Replacement

Replace when Phos <2.5 mg/dL

Electrolyte Replacement Principle

Correct all significant deficiencies

Potassium Phosphate Use

Useful when both potassium and phosphorus are low

Example Electrolyte Case

K 3.0, Mg 1.3, Phos 1.4 → Potassium phosphate + magnesium sulfate

Why Calcium Was Not Given in Example

Corrected calcium was normal after albumin adjustment

CKD Stage 5

Hemodialysis patients commonly develop anemia and CKD-MBD

Low Vitamin D Consequence

Reduced GI calcium absorption

Reduced GI Calcium Absorption

Result of low active vitamin D

High PTH Consequence

Increased bone resorption and bone disease

Secondary Hyperparathyroidism Goal

Reduce PTH while maintaining safe calcium and phosphorus levels

Calcitriol vs Cinacalcet

Calcitriol increases calcium; cinacalcet decreases calcium

Doxercalciferol Exception

Can be used despite normal vitamin D level for secondary hyperparathyroidism

Most Tested CKD-MBD Pattern

High phosphorus, low calcium, high PTH, low vitamin D

Most Tested ESA Concept

Risk increases when Hb exceeds 11 g/dL

Most Tested Iron Concept

TSAT <30% and ferritin <500 → give IV iron

Most Tested ABG Concept

Identify pH abnormality first, then determine respiratory vs metabolic cause

Most Tested Sodium Concept

Determine volume status before choosing treatment