Lecture 4: The Electrolytes - The Minor Ones

Bone Remodeling Cells

Osteoclasts perform bone breakdown, osteoblasts handle bone formation, and osteocytes nourish the skeleton and regulate bone activity.

Skeletal Electrolyte Stores

Bone holds 99% of body calcium, 85% of phosphorus, and 55% of magnesium.

Total Calcium Reference Range

8.6 – 10.3 mg/dL.

Blood Calcium Distribution

50% ionized (free), 40% protein-bound, and 10% complexed.

Hypocalcemia Cutoff & Most Common Cause

< 8.6 mg/dL; most commonly caused by low serum albumin.

Hypercalcemia Cutoff & Mnemonic

> 10.3 mg/dL; remembered by "Stones, bones, abdominal groans, and mental moans".

Spectrophotometric Calcium Assays

Arsenazo III (read at 650 nm, less Mg interference) and o-CPC (read at 570–580 nm).

Definitive Total Calcium Method

Isotope dilution-mass spectrometry (ID-MS).

Adjusted Calcium Formula

Adjusted Total Ca = Total Ca (mg/dL) + 0.8 × [4 – Albumin (g/dL)].

Ionized Calcium Reference Range & Method

4.6 – 5.3 mg/dL (1.15 – 1.33 mmol/L); measured via direct Ion-Selective Electrode (ISE).

Fasting Spot Urine Calcium Cutoff

> 0.16 mg/100 mL GF indicates increased osteoclastic activity.

Inorganic Phosphate Reference Range

2.5 – 4.5 mg/dL in adults (4.0 – 7.0 mg/dL in children).

Severe Hypophosphatemia Complication

< 1.0 mg/dL leads to metabolic encephalopathy due to ATP starvation in brain tissue.

Phosphate Assay Method

Photometric reaction with ammonium molybdate forming a phosphomolybdate complex read in UV at 340 nm.

Magnesium Reference Range & Function

1.7 – 2.4 mg/dL; essential cofactor for > 300 enzymes.

Therapeutic Magnesium Range

4.8 – 8.4 mg/dL (administered in pregnancy to prevent preeclampsia).

Magnesium Reference Method

Atomic Absorption Spectrometry (AAS).

PTH Synthesis & Trigger

Produced by chief cells in parathyroid glands in response to decreased ionized calcium.

Intact PTH Specimen & Reference Interval

EDTA plasma (plastic tube adsorption risk); reference interval is 10 – 65 pg/mL.

Primary Hyperparathyroidism Lab Pattern

Elevated serum calcium accompanied by elevated or inappropriately normal PTH.

Active Vitamin D Form

Calcitriol / 1,25(OH)2D, synthesized in the kidneys.

Preferred Vit D Clinical Marker & Range

25-hydroxyvitamin D [25(OH)D]; reference interval is 20 – 50 ng/mL (< 20 ng/mL indicates deficiency).

Gold Standard Vitamin D Assay

Liquid chromatography/tandem mass spectrometry (LC/MS/MS).