Phosphate Homeostasis, Assessment & Disorders

Felicity Stokes

Senior Clinical Biochemist

Royal Liverpool & Broadgreen University Hospital Trust

Talk Outline

• Introduction to phosphate

• Phosphate Homeostasis

• Assessment of phosphate status

• Disorders of phosphate homeostasis– Hypophosphataemia– Hyperphosphataemia

• Causes• Clinical Manifestations• Investigation• Management

Introduction• Phosphate ~ 23 mol in body

• Present in the body as Pi or organic phosphate

In the blood (ECF) 1%

Inorganic (Pi)

HPO42- & H2PO4

-

4 : 1

10% protein bound35% complexed with Ca2+/Mg2+

55% Free

Organic

Phospholipids etc

In cells (ICF) 14%

Inorganic (Pi)

Organic – Most PO4-

Intermediary metabolites

ATP

DNA

2,3 – biphosphoglycerate (RBCs)

85%

Hydroxyapatite crystals• ECF Pi measured – 0.8 – 1.5 mmol/L

(higher in children)

Functions• Mineralisation of bone (hydroxyapatite crystals with Ca2+)

• Formation of ATP from ADP

• Nucleic acid synthesis - DNA

• Intracellular metabolic pathways (glycolysis, pentose phosphate pathway, NADP)

• Phospholipids

• Oxygenation of tissues (2,3-biphosphoglycerate)

• Co-factor of enzyme reactions

• To act as a buffer in urine (HPO42- to H2PO4

-)

• Signalling pathways (cAMP, kinase cascades)

Phosphate Homeostasis

Intake

Excretion

1,25 Vitamin D

Absorbed from intestine

Plasma (1%)0.8 – 1.5 mmol/L

ICF (14%)Pi & organic phosphate

Excretion

Reabsorption by kidney into blood ~80%

PTH

PTH FGF23

Bone: 85% of total body phosphate

~45 mmol/day +

~ 19 mmol/day

~ 7 mmol/day

Present is a wide range of food, rare to have inadequate intake on Western diet

1,25 Vit D increases in low phosphate intake to increase amount that is absorbed

+

~ 7 mmol/day ~ 7 mmol/day

~ 33 mmol/day

-

~25 mmol/day

Renal Handling of phosphatePhosphate is reabsorbed primarily in proximal tubule by Na+/PO4

- co-transporters

~80% of phosphate filtered by glomerulus is reabsorbed

Phosphate excreted in urine is an important buffer

Na+ PO4-

PTH

FGF-23

• Parathyroid hormone (PTH)

• 1,25-dihydroxyvitamin D (active Vitamin D)

• Phosphatonins:

Fibroblast Growth Factor-23 (FGF-23)

Phosphate homeostasisMost foods phosphate rich. Therefore phosphate regulated mostly by altering renal excretion

PO4-

Ca2+

+

Phosphate homeostasis

PTH

Activation of Vitamin D

++ Reabsorption of Ca2+

Excretion of PO4-

Bone resorption to release Ca2+ & PO4

-

PO4- +

FGF-23

Absorption of Ca2+ and PO4-

PTH

PO4- & Ca2+ Ca2+ PO4

-

PTH

+

PO4- & Ca2+ PO4

-

+

-

FGF-23

Overall effect of PTH Ca2+ PO4-

Overall effect of FGF-23 PO4-

Assessment of phosphate status

Serum • U&Es – Renal function• Bone profile

– Calcium, phosphate, albumin, ALP• Parathyroid hormone (PTH)• 25-OH Vitamin D• Magnesium – nutritional status• FGF-23

Urine Urine phosphateCalculation of TmP/GFRAmino acids & glucose – fanconi syndrome

Hypophosphataemia

• Causes

• Signs & symptoms

• Investigation

• Management

Increased Loss

GI loss

Diarrhoea

Renal loss

Alcoholism – diuresis

Hyperparathyroidism

Fanconi syndrome

Post kidney transplant/dialysis

Hypophosphataemic rickets

Malignancy

Hypophosphatemia - Causes

Redistribution (into cells)Refeeding syndrome

Recovery from DKA

Alkalosis – Especially respiratory

ICF

Redistribution (into bone)Hungry bone syndrome

Plasma (0.8 – 1.5 mmol/L)

Inadequate intake/absorptionMalnutrition Alcoholism

Malabsorption

Vitamin D deficiency

Use of antacids

Redistribution

Plasma (0.8 – 1.5 mmol/L)

Re-feeding syndrome

Period of malnutrition, followed by intake of carbohydrates

Intracellular ions – Mg2+, PO4-, K+ which have leaked out of cells and been lost in

urine during malnutrition (causing a deficiency) are rapidly taken up by cells

Severe deficiency in plasma levels

All patients at risk of re-feeding are given:

Electrolytes BEFORE or WITH carbohydrate load

Mg2+

K+ PO4-Glucose

Glycolysis

Citric acid cycle

Oxidative phosphorylation

ATPInsulin

Redistribution

Also causes a decrease in plasma K+ and PO4-

after eating & treatment of DKA

FGF-23Most important phosphatonin

Synthesized by osteocytes of bone

251 amino acid peptide• Deactivated by enzymatic hydrolysis by PHEX at specific cleavage site

Activated by 1,25-OH Vitamin D and in turn deactivates 1,25-OH Vitamin D

Increased in:

• Renal failure/ dialysis patients

• High phosphate

• Hypophosphataemic rickets

Disorders of FGF-23Rare genetic disorders that lead to increased FGF-23

• X-linked hypophosphataemic rickets (XLH)Mutation in PHEX (enzyme that breaks down FGF-23 – coded for on X chromosome) decreasing FGF-23 hydrolysis

• Autosomal Dominant Hypophosphataemic Rickets (ADHR)

Mutation in FGF-23 at its cleavage site that prevents hydrolysis

• Oncogenic OsteomalaciaTumour cells that produce FGF-23

54 year old male with bowing of the tibia and femur, osteoarthritisPart of a family with known X-linked hypophosphataemic rickets

ACa2+ 2.36PO4

- 0.37

Post treatmentFGF-23 52 (<100)

Treatment

Alfacalcidol and phosphate tablets daily

Pre-treatmentFGF-23 222 (<100)

Low phosphate due to raised FGF-23

Causes ineffective mineralisation of bone – poor growth, fractures, bowing of limbs

PHEX

FGF-23FGF-23

FGF-23

FGF-23

FGF-23

Increased phosphate excretion in urine

FGF-23

ADHR

Hypophosphataemia - Signs & Symptoms

Acute• Muscle weakness – most common• Respiratory failure• Cardiac• Haematological• Reduced oxygenation of tissues

• Coma, convulsions & death – very severe

Chronic• Rickets – children

• Osteomalacia - adults

Mild hypophosphatemia has minimal clinical signs & symptoms, but severe may be associated by profound complications

Hypophosphataemia - Investigation1. RepeatPhosphate varies throughout day – transcellular shift (with insulin & glucose) also exclude transient cause – respiratory alkalosis

2. ?Serum ACa2+

Both affected by PTH & vitamin D

3. ?Serum Mg2+, PO4-, K+

All intracellular ions, low in alcoholism & re-feeding

4. Further investigation

Urine phosphate

High Ca2+ Hyperparathyroidism

Low Ca2+ Vit D deficiency

All lowAlcoholism/ re-feeding/ nutritional deficiency

Low urine PO4-

High urine PO4-

Appropriate

Renal loss

Urine amino acids & glucoseHighFanconi syndrome

25-OHVit D

PTH

Measure

Measure

FGF-23 High

XLHADHRTIO

Blood gas

Urine Phosphate• Paired fasting 2nd void urine & blood for calculation of TmP/GFR

• If borderline results – fasting 2hr urine collection with bloods in middle

• TmP/GFR useful for investigation of hypophosphataemia – determine whether the cause is renal loss

FE =UPhos x PCreat

UCreat x PPhos

TmP/GFRCalculate fractional excretion (FE) of phosphate:

The renal tubular maximum reabsorption of phosphate per litre of GFR

TmP/GFR = TR x [plasma phosphate]

1-FE = TR (tubular reabsorption)

TR = Fraction of filtered PO4- that is reabsorbed

To convert to a concentration and standardise per volume of filtrate TmP/GFR

TmP/GFR = 1.6 mmol/L

Management

Contraindications to phosphate replacement

• Hypocalcaemia (will bind Ca2+ & decrease it further)• Pancreatitis – Calcium may be low due to saponification in damaged tissue • Can’t give iv with magnesium or calcium as will precipitate – administer in separate arms• Renal failure! – give a reduced amount (usually halved)

16.1 mmol PO4-

20.4 mmol Na+ 3.1 mmol K+

Oral phosphate replacementPhosphate–Sandoz soluble tablets Give up to 100mmol (6 tablets) of phosphateSide effects - diarrhoea

Intravenous phosphate replacement – acute or severe/NBM20 mmol in 500mL over 12 hoursPotassium acid phosphatePolyfusor

Treat the underlying cause – eg. Replace if Vitamin D deficient

Replace low PO4- & maintenance PO4

- when known depletion/losses

Monitor serum Ca2+, PO4-, Mg2+ and creatinine

20 mmol K+ & 20 mmol PO4- in 250ml

50 mmol PO43-, 9.5 mmol K+, 81 mmol Na+ in 500mL Discard after 20 mmol (200mL)

Hyperphosphataemia

• Causes– Pseudohyperphosphataemia

• Signs & symptoms

• Investigation

• Management

Hyperphosphataemia - Causes

Increased exogenous loadi.v. infusion

Vitamin D toxicity

Phosphate-containing enemas

Redistribution (from cells)Tumour lysis syndrome

Rhabdomyolysis

In vivo haemolysis

Acidosis – esp Lactic acidosis

Decreased Urinary ExcretionRenal failure – AKI/ CKD

Hypoparathyroidism

PseudohyperphosphatemiaHaemolysis

Delayed separation

Plasma (0.8 – 1.5 mmol/L)

Pseudohyperphosphataemia

• Redistribution from cells– Haemolysis – results should be automatically knocked out from LIMS & not

reported

– Delayed separation – check date of sample, especially in GP samples. Especially if no obvious cause of PO4

-/other abnormalities except a K+ due to same mechanism

• Paediatrics – make sure you are using age-related reference ranges

[Plasma phosphate] high in neonates & falls progressively throughout childhood

64 year old lady with known multiple myeloma

Routine bloods before a bisphosphonate infusion

Sodium 144 Potassium 4.3 Chloride 102 Bicarb. 28 Urea 4.9 Creatinine 55 eGFR >90

Adj Ca 2.45 Phosphate 3.16 Calcium 2.38Albumin 39Protein 92Globulin 53ALP 46

High protein and globulins due to paraprotein

Very high phosphate - ?Cause

1. Rule out spurious – not old or haemolysed

2. Normal renal function

3. Normal calcium – not hypoparathyroidism

4. Normal K+ and no clinical details to suggest cell lysis – rhabdomyolysis/ tumour lysis

5. If no obvious clinical causes – think rare causes/ interference

Paraproteins previously reported to cause interference in phosphate assays

Usually IgM – as these are large molecules and can interfere with the absorbance. Diluting the sample may dilute out some of the interfering immunoglobulins

This patient had an IgG paraprotein

Sample diluted 1 in 3 - result = 0.34 mmol/L

Multiply by 3 to correct for dilution factor = 1.02 mmol/L

Sample had an abnormal reaction profile

Patient has a normal phosphate concentration

Sodium 153Potassium 4.4Chloride 99Bicarb. 14Urea 25.2Creatinine 166eGFR 25

Adj Ca 1.54Phosphate >11.00Calcium 1.46Albumin 38Protein 61Globulin 23Alk Phos 88

Magnesium 1.00

CRP 96

ALT 27Alk Phos 88Bilirubin 18GGT 64

85 year old lady with bowel obstruction. Results were phoned to ward overnight by lab

Main abnormalities:

Renal failure

Very low ACa2+

Very high PO4-

?Genuine/ ?Spurious - ?Cause

Results from the previous day

Sodium 139Potassium - 2.6Chloride 95Bicarb. 19Urea 20.0Creatinine 127eGFR 35

Adj Ca 1.94Phosphate 4.10Albumin 37Protein 63Globulin 26

Look genuine as results were similar previous day

What could cause such deranged results? Adj Ca 1.54Phosphate >11.00Calcium 1.46Albumin 38Protein 61Globulin 23Alk Phos 88

Patient had a bowel obstruction and was given repeated phosphate enemas to clear it

Reduced renal function means phosphate could not be cleared

High phosphate – complexed with Ca2+ and caused low Ca2+

Mainly due to binding with calcium

Acute– Hypocalcaemia

• Tetany, pins & needle

Chronic– Precipitation - calcification of blood vessels & soft tissues

• Renal failure• Cardiovascular complications

Signs & Symptoms

Hyperphosphataemia - Investigation1. ?GenuineExclude age-related (children have higher [PO4

-]), delayed separation, haemolysis and iatrogenic (over-replacement)

2. ?Renal functionRenal failure most common cause of PO4

-

U&Es – high urea & creat AKI/CKD

3. ?Serum ACa2+

Both affected by PTH & Vitamin D

High Ca2+ Vitamin D toxicity, malignancy

Measure

25-OH Vit D

Low Ca2+ Hypoparathyroidism PTH & Mg2+

4. Urine PO4- Low

High AppropriateRedistribution – cell lysisIncreased intake Malignancy

5. Markers of cell lysisCK, urate High Rhabdomyolysis/ Tumour lysis syndrome

Normal Ca2+ Lactic acidosis Lactate

Management

Chronic• Dietary restriction of phosphate – difficult – present in lots of foods• Administration of Phosphate binders

– Selevamer– Calcium - hypercalcaemia– Aluminium hydroxide – aluminium toxicity

Acute• Aggressive fluid hydration• Administration of insulin & dextrose – only temporary• Haemodialysis

Treat the underlying cause

Thank you

Any questions?