Total calcium 7.4 mg/dL (1.85 mmol/L; reference 8.6-10.3 mg/dL, 2.15-2.57 mmol/L), albumin 2.0 g/dL (20 g/L), ionized calcium 4.9 mg/dL (1.22 mmol/L; reference 4.6-5.3 mg/dL, 1.15-1.32 mmol/L). How is the low total calcium best explained?

A low-albumin effect with normal ionized calcium

True hypocalcemia needing calcium replacement

Hypoparathyroidism with low ionized calcium

Calcium chelation from EDTA contamination

PTH acts on the kidney to:

Raise calcium reabsorption and phosphate excretion

Raise both calcium and phosphate reabsorption

Lower calcium reabsorption and phosphate excretion

Raise calcium excretion and phosphate reabsorption

Electrolytes: calcium, magnesium, phosphorus

9 ASCP MLS practice questions on Electrolytes: calcium, magnesium, phosphorus, part of Chemistry. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
Which form of serum calcium is physiologically active and controls PTH secretion?
1. ACitrate-complexed calcium
2. BAlbumin-bound calcium
3. CGlobulin-bound calcium
4. DFree ionized calcium
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Answer: D · Free ionized calcium
About 45-50% of serum calcium is free and ionized, about 40-45% is bound to proteins (mainly albumin), and about 10% is complexed with anions such as phosphate, citrate and bicarbonate. Only the ionized fraction acts on nerves, muscle and clotting, and the calcium-sensing receptor on parathyroid cells responds to it. Total calcium can therefore mislead when albumin or pH is abnormal. Ionized calcium is measured directly with an ion-selective electrode.
- A: Complexed calcium is about 10% of the total and is not the regulated fraction.
- B: Albumin-bound calcium is an inactive reservoir that follows the albumin level.
- C: Globulins carry only a small share of bound calcium, and it is inactive.
Question 2 · Easy
Which hormone, made by the C cells of the thyroid, lowers serum calcium by inhibiting osteoclasts?
1. ACalcitriol
2. BCalcitonin
3. CParathyroid hormone
4. DPTH-related protein
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Answer: B · Calcitonin
Calcitonin is secreted by the parafollicular C cells when calcium rises, and it reduces bone resorption by inhibiting osteoclasts. Its role in day-to-day calcium control in adults is small compared with PTH and vitamin D. In the laboratory it is used mainly as a tumor marker for medullary thyroid carcinoma, which arises from C cells. PTH, calcitriol and PTHrP all raise serum calcium.
- A: Calcitriol is made in the kidney and raises calcium absorption from the gut.
- C: PTH comes from the parathyroid glands and raises calcium.
- D: PTHrP is made by some tumors and raises calcium as PTH does.
Question 3 · Medium
Total calcium 7.4 mg/dL (1.85 mmol/L; reference 8.6-10.3 mg/dL, 2.15-2.57 mmol/L), albumin 2.0 g/dL (20 g/L), ionized calcium 4.9 mg/dL (1.22 mmol/L; reference 4.6-5.3 mg/dL, 1.15-1.32 mmol/L). How is the low total calcium best explained?
1. ATrue hypocalcemia needing calcium replacement
2. BA low-albumin effect with normal ionized calcium
3. CHypoparathyroidism with low ionized calcium
4. DCalcium chelation from EDTA contamination
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Answer: B · A low-albumin effect with normal ionized calcium
About 40% of serum calcium is protein-bound, mostly to albumin, so a low albumin lowers total calcium without changing the free fraction. The ionized calcium is within its reference range, so calcium balance is intact and no replacement is needed. A common correction adds 0.8 mg/dL of calcium for each 1 g/dL that albumin is below 4.0 g/dL (here 7.4 + 1.6 = 9.0 mg/dL), but a measured ionized calcium is more reliable. EDTA contamination would lower the ionized calcium as well and raise potassium.
- A: The ionized calcium is normal, so replacement is not needed.
- C: Hypoparathyroidism lowers ionized calcium, which is normal here.
- D: EDTA would lower the ionized calcium as well and raise potassium.
Question 4 · Medium
A man with chronic alcohol use has hypocalcemia with an inappropriately low PTH, and it does not correct with IV calcium. Which result should be checked next?
1. AChloride
2. BMagnesium
3. CPhosphate
4. DPotassium
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Answer: B · Magnesium
Severe magnesium deficiency impairs PTH secretion and makes bone and kidney resistant to PTH, so hypocalcemia persists until magnesium is replaced. Alcohol use, malabsorption, diuretics and proton pump inhibitors are common causes of low magnesium. Hypomagnesemia also causes potassium loss that is hard to correct. Magnesium should be checked in any hypocalcemia that responds poorly to treatment.
- A: Chloride has no direct role in PTH secretion or calcium balance.
- C: A high phosphate can lower calcium but does not block PTH secretion.
- D: Low potassium does not suppress PTH or cause resistant hypocalcemia.
Question 5 · Easy
PTH acts on the kidney to:
1. ARaise calcium reabsorption and phosphate excretion
2. BRaise both calcium and phosphate reabsorption
3. CLower calcium reabsorption and phosphate excretion
4. DRaise calcium excretion and phosphate reabsorption
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Answer: A · Raise calcium reabsorption and phosphate excretion
PTH increases calcium reabsorption in the distal tubule and blocks phosphate reabsorption in the proximal tubule, so phosphate is lost in the urine. It also stimulates renal 1-alpha-hydroxylase, which makes calcitriol and raises calcium absorption from the gut. That is why primary hyperparathyroidism gives a high calcium with a low or low-normal phosphate. In kidney failure, phosphate cannot be excreted and rises despite a high PTH.
- B: PTH reduces phosphate reabsorption, which causes phosphaturia.
- C: PTH increases, not decreases, calcium reabsorption.
- D: That is the reverse of both PTH effects on the tubule.
Question 6 · Medium
A 24-year-old woman breathing rapidly during a panic attack has tingling around the mouth and carpopedal spasm. Her total calcium is normal. The most likely explanation is:
1. AAcidosis has lowered her ionized calcium
2. BAlkalosis has raised her serum phosphate
3. CAlkalosis has lowered her ionized calcium
4. DAlkalosis has lowered her serum albumin
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Question 7 · Medium
A heparinized whole-blood sample for ionized calcium is left uncapped for 30 minutes before analysis. The result will be:
1. AFalsely low, as CO2 loss raises the pH
2. BUnchanged, as total calcium is unchanged
3. CFalsely high, as CO2 loss raises the pH
4. DFalsely low, as CO2 loss lowers the pH
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Question 8 · Hard
Calcium 8.1 mg/dL (2.02 mmol/L; reference 8.6-10.3 mg/dL), phosphate 6.9 mg/dL (2.23 mmol/L; reference 2.5-4.5 mg/dL), creatinine 5.6 mg/dL (495 µmol/L), PTH 410 pg/mL (410 ng/L; reference 15-65) and a raised alkaline phosphatase. These results are most consistent with:
1. APseudohypoparathyroidism
2. BSecondary hyperparathyroidism
3. CHypoparathyroidism
4. DPrimary hyperparathyroidism
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Question 9 · Hard
Raised serum calcium, raised PTH and low phosphate most suggest which condition?
1. AHypercalcemia of malignancy from PTHrP
2. BChronic kidney disease
3. CPrimary hyperparathyroidism
4. DVitamin D deficiency
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Electrolytes: calcium, magnesium, phosphorus

Which form of serum calcium is physiologically active and controls PTH secretion?

Which hormone, made by the C cells of the thyroid, lowers serum calcium by inhibiting osteoclasts?

Total calcium 7.4 mg/dL (1.85 mmol/L; reference 8.6-10.3 mg/dL, 2.15-2.57 mmol/L), albumin 2.0 g/dL (20 g/L), ionized calcium 4.9 mg/dL (1.22 mmol/L; reference 4.6-5.3 mg/dL, 1.15-1.32 mmol/L). How is the low total calcium best explained?

A man with chronic alcohol use has hypocalcemia with an inappropriately low PTH, and it does not correct with IV calcium. Which result should be checked next?

PTH acts on the kidney to:

A 24-year-old woman breathing rapidly during a panic attack has tingling around the mouth and carpopedal spasm. Her total calcium is normal. The most likely explanation is:

A heparinized whole-blood sample for ionized calcium is left uncapped for 30 minutes before analysis. The result will be:

Calcium 8.1 mg/dL (2.02 mmol/L; reference 8.6-10.3 mg/dL), phosphate 6.9 mg/dL (2.23 mmol/L; reference 2.5-4.5 mg/dL), creatinine 5.6 mg/dL (495 µmol/L), PTH 410 pg/mL (410 ng/L; reference 15-65) and a raised alkaline phosphatase. These results are most consistent with:

Raised serum calcium, raised PTH and low phosphate most suggest which condition?