Disorders of Phosphorus

What is the importance of phosphorus?

Essential element in cell membrane, nucleic acids, and phosphoproteins

Results metabolism of carbs/fats/proteins and enzymatic reactions

Required for normal O2/Hgb dissociation

Source of high energy bonds in ATP

What is the range of normal serum phosphorus?

2.8-4.5 mg/dL

What are key regulators of phosphorus?

Influx via GI and bone

Tubular reabsorption

Define hypophosphatemia

Phosphorus < 2.8 mg/dL

Define mild-moderate hypophosphatemia

1.5-2.7 mg/dL

Define severe hypophosphatemia

< 1.5 mg/dL

How can reduced GI absorption cause hypophosphatemia?

Phosphate binding drug, decreased intake, glucocorticoids, vitamin B deficiency, hyperparayhtroidism, chronic diarrhea

How can reduced tubular reabsorption cause hypophosphatemia?

Hyperparathyroidism, recovery from extensive burns, rickets, malignancy, vitamin D deficiency, diuretics, glucocorticoids, bicarbonate

How can a transcellular shift (ECF to ICF) cause hypophosphatemia?

Refeeding syndrome, parathyroidectomy, alcohol use disorder, respiratory alkalosis, DKA treatment, catecholamines, anabolic steroids, calcitonin

Describe the clinical presentation of mild-moderate hypophosphatemia

Rarely symptomatic

Irritability, apprehension, weakness, numbness, paresthesia, confusion

Severe, acute development may result in seizures or coma

Describe the clinical presentation of severe hypophosphatemia

Initial response of bone = hypercalcemia (if prolonged = rickets and/or osteomalacia)

Metabolic encephalopathy, impaired myocardial contractility/respiratory failure, rhabdomyolysis, hemolysis

What are the treatment goals of hypophosphatemia?

Reverse signs/symptoms, normalize phos levels, address underlying causes

How should mild-moderate hypophosphatemia be treated?

Oral phosphate salts

IV phosphate if severe or unable to tolerate oral

How should mild-moderate hypophosphatemia be monitored with oral phosphate salts?

Monitor phosphate daily

What is the goal of treatment of mild-moderate hypophosphatemia with oral phosphate salts?

Goal to replete phosphorus in 7-10 days

What are AEs of oral phosphate salts?

Osmotic diarrhea

How is IV phosphate available?

As Na and K salts

Kphos has about 22 mEq of K per 15 mmol phosphate

What are common dosages of IV phosphate?

15 mmol and 30 mmol

What is the maximum infusion rate of IV phosphate and why?

7.5 mmol/hour

Reduces the risk of hypocalcemia, hypotension, metastasis calcification, or renal failure

When should IV phosphate doses be reduced?

Reduce dose by 50% with impaired renal function

What should be used in IV phosphate dosing in obesity and why?

AdjBW to avoid over dosing

Define hyperphosphatemia

Phosphorous > 4.5 mg/dL

What is hyperphosphatemia usually a result of?

Renal dysfunction or endogenous intracellular release of phosphorus

How can renal dysfunction (CKD, AKI) cause hyperphosphatemia?

Decreased tubular excretion

How can hypoparathyroidism cause hyperphosphatemia?

Increased tubular reabsorption

What are examples of rapid tissue catabolism that can cause hyperphosphatemia?

Necrosis of skeletal muscle, tumor lysis syndrome

What are examples of acid-base disorders that can cause hyperphosphatemia?

Lactic acidosis, DKA

What are examples of exogenous phosphate loads that can cause hyperphosphatemia?

Phosphorous containing IV, oral, and rectal products

Describe the clinical presentation of hyperphosphatemia

GI disturbances, lethargy, urinary obstruction, seizures (rarely)

Hypocalcemia, calciphylaxis

What are the treatment goals of hyperphosphatemia?

Avoid GI and neuro symptoms, prevent disposition in urinary tract to avoid AKI, return phosphate to normal (or near normal in ESRD), prevent calcification of vasculature

How should severe/symptomatic hyperphosphatemia manifesting as hypocalcemia and tetany be treated?

IV calcium

If calcium is not critically low, how should hyperphosphatemia be treated?

Limit phos intake and block absorption

Can consider hemodialysis if still symptomatic

What is the primary treatment of hyperphosphatemia?

Phosphate binders

What is the MOA of phosphate binders?

Limit GI absorption of phosphorous

What phosphate binders should be avoided and why?

Avoid aluminum based binders due to risk of aluminum accumulation and toxicity