Calcium and Magnesium, the Mirror Pair
A symptom-distribution body map: one simplified anterior figure split down the midline, left half labeled LOW carrying perioral tingling, facial twitch, carpal spasm, and a brisk patellar reflex, right half labeled HIGH carrying lethargy, constipation, an absent patellar reflex, and a reduced-respiration marker at the chest, with a key separating calcium sites from magnesium sites by solid and hollow dots.
| CALCIUM total 9.0–10.5 mg/dL, ionized 4.5–5.6 mg/dL · critical below 6.0 or above 13.0 mg/dL | MAGNESIUM 1.3–2.1 mEq/L · critical below 1.0 or above 4.7 mEq/L | |
|---|---|---|
| PUSHES IT UP | Immobility, excess intake, bone breakdown, thiazide therapy | Renal failure, magnesium-containing antacids and laxatives, magnesium therapy |
| PUSHES IT DOWN | Removal or suppression of parathyroid function, low vitamin D, low magnesium, massive transfusion | Alcohol use, malnutrition, GI losses, loop diuretics |
| WHAT THE BODY DOES | Pulls calcium from bone and reabsorbs it at the tubule when the level falls | Excretes magnesium at the tubule, so a failing kidney cannot unload it |
| WHAT YOU SEE, LOW | Irritability: tingling around the mouth and fingers, twitching, a positive Chvostek sign, a positive Trousseau sign, laryngospasm | Irritability: hyperreflexia, tremor, and the rhythm risk that makes magnesium the value you check before chasing a potassium that will not correct |
| WHAT YOU SEE, HIGH | Sedation: lethargy, weak muscle tone, constipation, bone pain, stones | Sedation: deep tendon reflexes disappear first, then the respiratory rate falls |
| FIRST ACTION, LOW | Airway and seizure precautions before replacement | Check the potassium alongside it, because one will not correct without the other |
| FIRST ACTION, HIGH | Fluids and mobility, and hold the calcium source | Stop the source, check deep tendon reflexes and respiratory rate, and have calcium gluconate available |
| CRITICAL CALL | Below 6.0 or above 13.0 mg/dL: airway assessment, then call | Below 1.0 or above 4.7 mEq/L: reflexes and respiratory rate, then call |
Shadow row: phosphate 3.0 to 4.5 mg/dL. Phosphate moves opposite calcium. Read one and you can predict the other before the second result posts.
Margin notehigh sedates, low irritates.
Watch outin magnesium excess the deep tendon reflexes go absent before the respirations fall. Losing the reflex is the warning, not the aftermath. Check reflexes on a schedule, not on a hunch.
Memory hookCALCIUM AND MAGNESIUM RIDE TOGETHER. Both sedate when high and irritate when low, so one pattern covers four directions.
☐ I can predict the sign from the direction alone, before I know the cause.
→ BOOK 10 · P7 for parathyroid and kidney conditions. → BOOK 05 · P11 for the calcium gluconate profile and → BOOK 06 · P9 for its math.
Takeawaythe direction predicts the neuromuscular picture before you know the cause.
Reference ranges and critical thresholds vary between institutions and reference laboratories. Follow local protocol.