An 81-year-old woman, admitted with a hip fracture after a fall, is poorly nourished. Results: total calcium 2.42 mmol/L (9.7 mg/dL; reference 2.15-2.55), albumin 24 g/L (2.4 g/dL; reference 35-50), phosphate 0.72 mmol/L (2.2 mg/dL; reference 0.80-1.50), intact PTH 11.5 pmol/L (108 ng/L; reference 1.6-6.9), creatinine 66 µmol/L (0.75 mg/dL) and 25-hydroxyvitamin D 68 nmol/L (27 ng/mL; sufficient above 50). The laboratory corrects calcium as measured calcium + 0.02 x (40 - albumin in g/L). Her corrected calcium and the diagnosis the results suggest are:

2.74 mmol/L; primary hyperparathyroidism

2.42 mmol/L; normal calcium, low albumin

2.10 mmol/L; secondary hyperparathyroidism

2.74 mmol/L; hypercalcemia of malignancy

In the o-cresolphthalein complexone method for total calcium, 8-hydroxyquinoline is added to:

Stop magnesium from reacting with the dye

Release calcium bound to albumin

Keep the reaction mixture at acid pH

Remove phosphate from the sample

Why can a serum magnesium within the reference range fail to exclude total-body magnesium depletion?

Less than 1% of body magnesium is extracellular

Clotting removes magnesium from the serum

Assays measure only the free ionized fraction

The kidney cannot reduce magnesium excretion

During surgery for a single parathyroid adenoma, intact PTH is 18.0 pmol/L (170 ng/L) just before excision and 7.2 pmol/L (68 ng/L) 10 minutes after the gland is removed (reference 1.6-6.9 pmol/L). Using the criterion of a fall of more than 50% from the pre-excision value, the result:

Shows a 60% fall, predicting a successful cure

Shows only a 40% fall; another gland is likely overactive

Cannot be judged, as PTH lasts hours in plasma

Fails, as PTH is still above the reference limit

Calcium, phosphate and magnesium

13 DOH Abu Dhabi (HAAD) lab licensing practice questions on Calcium, phosphate and magnesium, part of Clinical Chemistry. The first 13 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 · Hard
An 81-year-old woman, admitted with a hip fracture after a fall, is poorly nourished. Results: total calcium 2.42 mmol/L (9.7 mg/dL; reference 2.15-2.55), albumin 24 g/L (2.4 g/dL; reference 35-50), phosphate 0.72 mmol/L (2.2 mg/dL; reference 0.80-1.50), intact PTH 11.5 pmol/L (108 ng/L; reference 1.6-6.9), creatinine 66 µmol/L (0.75 mg/dL) and 25-hydroxyvitamin D 68 nmol/L (27 ng/mL; sufficient above 50). The laboratory corrects calcium as measured calcium + 0.02 x (40 - albumin in g/L). Her corrected calcium and the diagnosis the results suggest are:
1. A2.42 mmol/L; normal calcium, low albumin
2. B2.10 mmol/L; secondary hyperparathyroidism
3. C2.74 mmol/L; primary hyperparathyroidism
4. D2.74 mmol/L; hypercalcemia of malignancy
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Answer: C · 2.74 mmol/L; primary hyperparathyroidism
Corrected calcium = 2.42 + 0.02 x (40 - 24) = 2.42 + 0.32 = 2.74 mmol/L (11.0 mg/dL), so she is hypercalcemic although her total calcium looks normal. A raised PTH alongside a raised calcium is inappropriate and points to autonomous parathyroid secretion, and the low phosphate fits the phosphate-wasting action of PTH on the kidney. A normal creatinine and a sufficient 25-hydroxyvitamin D argue against secondary hyperparathyroidism, which has a low or normal calcium. Malignancy, whether from bone metastases, myeloma or PTHrP, suppresses PTH. Because the correction is only an estimate when albumin is very low, an ionized calcium is the best confirmation.
- A: This uses the measured total calcium and ignores the very low albumin.
- B: This subtracts the correction; a low albumin makes the corrected calcium higher than the measured value.
- D: The calculation is right, but hypercalcemia of malignancy suppresses PTH instead of raising it.
Question 2 · Easy
In the o-cresolphthalein complexone method for total calcium, 8-hydroxyquinoline is added to:
1. ARelease calcium bound to albumin
2. BKeep the reaction mixture at acid pH
3. CStop magnesium from reacting with the dye
4. DRemove phosphate from the sample
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Answer: C · Stop magnesium from reacting with the dye
o-Cresolphthalein complexone forms a red-purple complex with calcium at alkaline pH, read near 575 nm. Magnesium forms a similar colored complex, so 8-hydroxyquinoline is added to bind magnesium and stop it from reacting. Arsenazo III is another dye widely used for total calcium on automated analyzers. Atomic absorption spectrophotometry is the reference method for total calcium.
- A: Protein-bound calcium is released by the reagent conditions, not by 8-hydroxyquinoline.
- B: The reaction runs at alkaline pH, around 10 or above.
- D: Phosphate is not removed; 8-hydroxyquinoline targets magnesium.
Question 3 · Medium
Why can a serum magnesium within the reference range fail to exclude total-body magnesium depletion?
1. ALess than 1% of body magnesium is extracellular
2. BClotting removes magnesium from the serum
3. CAssays measure only the free ionized fraction
4. DThe kidney cannot reduce magnesium excretion
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Answer: A · Less than 1% of body magnesium is extracellular
About half of body magnesium is in bone and nearly all the rest is inside cells, so less than 1% circulates in plasma. Bone and cells can release magnesium to hold the serum level up while total stores fall, so serum magnesium may be normal despite significant depletion. A low 24-hour urine magnesium with normal kidney function, or a magnesium loading (retention) test, supports deficiency. Hypokalemia or hypocalcemia that will not correct should prompt a magnesium check even when serum magnesium looks normal.
- B: Serum and plasma magnesium are almost the same; clotting does not remove it.
- C: Routine colorimetric assays measure total serum magnesium, not only the free fraction.
- D: The kidney cuts magnesium excretion sharply in deficiency, which is why urine magnesium helps.
Question 4 · Hard
During surgery for a single parathyroid adenoma, intact PTH is 18.0 pmol/L (170 ng/L) just before excision and 7.2 pmol/L (68 ng/L) 10 minutes after the gland is removed (reference 1.6-6.9 pmol/L). Using the criterion of a fall of more than 50% from the pre-excision value, the result:
1. AShows only a 40% fall; another gland is likely overactive
2. BShows a 60% fall, predicting a successful cure
3. CCannot be judged, as PTH lasts hours in plasma
4. DFails, as PTH is still above the reference limit
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Answer: B · Shows a 60% fall, predicting a successful cure
The fall is (18.0 − 7.2) ÷ 18.0 = 10.8 ÷ 18.0 = 60%, which meets the criterion of a drop of more than 50% at 10 minutes after excision. Intact PTH has a half-life of only a few minutes, so removing the only overactive gland makes the level fall quickly. Under this criterion the value does not have to be within the reference range at 10 minutes, because the remaining PTH is still being cleared. A fall of less than 50% suggests another overactive gland and prompts further exploration.
- A: 7.2 is 40% of the starting value, so the fall is 60%.
- C: PTH has a half-life of only a few minutes, which is why it is used during surgery.
- D: The criterion uses the percentage fall, not a return to the reference range.
Question 5 · Hard
In mild hypercalcemia with a PTH in the upper reference range: serum calcium 2.70 mmol/L, serum creatinine 81 µmol/L, and on the same 24-hour urine, calcium 1.0 mmol/L and creatinine 10 mmol/L. The calcium/creatinine clearance ratio = (urine Ca × serum creatinine) ÷ (serum Ca × urine creatinine); below 0.01 favors familial hypocalciuric hypercalcemia (FHH), above 0.02 primary hyperparathyroidism (PHPT). What is the ratio and its meaning?
1. A0.003, favoring FHH
2. B3.0, favoring PHPT
3. C0.003, favoring PHPT
4. D0.03, favoring PHPT
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Answer: A · 0.003, favoring FHH
Serum creatinine must be in the same units as urine creatinine: 81 µmol/L = 0.081 mmol/L. The ratio is (1.0 × 0.081) ÷ (2.70 × 10) = 0.081 ÷ 27 = 0.003, well below 0.01. In FHH, an inactivating variant of the calcium-sensing receptor makes the kidney reabsorb calcium avidly, so urine calcium stays low despite hypercalcemia. FHH is benign and needs no parathyroid surgery, so separating it from PHPT matters.
- B: 3.0 comes from leaving serum creatinine in µmol/L while urine creatinine is in mmol/L.
- C: A ratio below 0.01 favors FHH, not PHPT.
- D: 0.03 comes from converting 81 µmol/L to 0.81 instead of 0.081 mmol/L.
Question 6 · Medium
Two days after a total thyroidectomy, a woman has tingling around the mouth. Calcium is 1.82 mmol/L (7.3 mg/dL; reference 2.15-2.55), phosphate 1.75 mmol/L (5.4 mg/dL; reference 0.80-1.50), magnesium is normal and PTH is undetectable. What is the most likely diagnosis?
1. AHypoparathyroidism
2. BSecondary hyperparathyroidism
3. CPseudohypoparathyroidism
4. DVitamin D deficiency
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Question 7 · Medium
A grossly hemolyzed serum sample is analyzed for minerals. Which result is falsely increased because red cells contain much more of it than plasma?
1. AIonized calcium
2. BSodium
3. CMagnesium
4. DTotal calcium
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Question 8 · Hard
A septic ICU patient has total calcium 1.90 mmol/L (7.6 mg/dL) and albumin 18 g/L (1.8 g/dL), giving an albumin-corrected calcium of 2.34 mmol/L (reference 2.15-2.55). A correctly collected ionized calcium is 0.98 mmol/L (reference 1.15-1.32). The best interpretation is:
1. ALow albumin alone explains the low total calcium
2. BTrue hypocalcemia; the ionized result should guide care
3. CNormocalcemia; the corrected value should guide care
4. DThe ionized result is wrong and must be repeated
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Question 9 · Easy
In a healthy adult, about what share of total plasma calcium is free ionized calcium?
1. AAbout 50%
2. BAbout 95%
3. CAbout 80%
4. DAbout 10%
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Question 10 · Hard
Results: calcium 1.85 mmol/L (7.4 mg/dL; reference 2.15-2.55), phosphate 2.05 mmol/L (6.3 mg/dL; reference 0.80-1.50), intact PTH 24 pmol/L (226 ng/L; reference 1.6-6.9), magnesium 0.85 mmol/L (reference 0.70-1.00), creatinine 58 µmol/L (0.66 mg/dL) and 25-hydroxyvitamin D 75 nmol/L (30 ng/mL). Which diagnosis do these results fit best?
1. APrimary hypoparathyroidism
2. BPseudohypoparathyroidism
3. CVitamin D deficiency
4. DSecondary hyperparathyroidism of CKD
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Question 11 · Medium
A 64-year-old man with a lung mass has calcium 3.20 mmol/L (12.8 mg/dL; reference 2.15-2.55), phosphate 0.65 mmol/L (2.0 mg/dL; reference 0.80-1.50) and intact PTH 0.6 pmol/L (6 ng/L; reference 1.6-6.9). A bone scan shows no metastases. The most likely mechanism is:
1. ABone destruction by lytic tumor deposits
2. BCalcitriol production by the tumor tissue
3. CAutonomous secretion by a parathyroid adenoma
4. DTumor secretion of PTH-related peptide
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Question 12 · Medium
During a massive transfusion, total calcium is 2.20 mmol/L (8.8 mg/dL; reference 2.15-2.55) but ionized calcium, measured with balanced heparin, is 0.86 mmol/L (reference 1.15-1.32). Why are they so different?
1. APotassium from stored red cells has displaced it
2. BAcidosis has moved calcium onto albumin
3. CDilution has lowered the albumin-bound calcium
4. DCitrate from the units has bound the ionized calcium
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Question 13 · Hard
Total calcium is 7.6 mg/dL and albumin is 2.0 g/dL. What is the albumin-corrected calcium (using 4.0 g/dL as normal albumin)?
1. A5.6 mg/dL
2. B9.2 mg/dL
3. C7.6 mg/dL
4. D8.4 mg/dL
Answer and explanation in LearnFree account

Calcium, phosphate and magnesium

In the o-cresolphthalein complexone method for total calcium, 8-hydroxyquinoline is added to:

Why can a serum magnesium within the reference range fail to exclude total-body magnesium depletion?

During surgery for a single parathyroid adenoma, intact PTH is 18.0 pmol/L (170 ng/L) just before excision and 7.2 pmol/L (68 ng/L) 10 minutes after the gland is removed (reference 1.6-6.9 pmol/L). Using the criterion of a fall of more than 50% from the pre-excision value, the result:

Two days after a total thyroidectomy, a woman has tingling around the mouth. Calcium is 1.82 mmol/L (7.3 mg/dL; reference 2.15-2.55), phosphate 1.75 mmol/L (5.4 mg/dL; reference 0.80-1.50), magnesium is normal and PTH is undetectable. What is the most likely diagnosis?

A grossly hemolyzed serum sample is analyzed for minerals. Which result is falsely increased because red cells contain much more of it than plasma?

A septic ICU patient has total calcium 1.90 mmol/L (7.6 mg/dL) and albumin 18 g/L (1.8 g/dL), giving an albumin-corrected calcium of 2.34 mmol/L (reference 2.15-2.55). A correctly collected ionized calcium is 0.98 mmol/L (reference 1.15-1.32). The best interpretation is:

In a healthy adult, about what share of total plasma calcium is free ionized calcium?

A 64-year-old man with a lung mass has calcium 3.20 mmol/L (12.8 mg/dL; reference 2.15-2.55), phosphate 0.65 mmol/L (2.0 mg/dL; reference 0.80-1.50) and intact PTH 0.6 pmol/L (6 ng/L; reference 1.6-6.9). A bone scan shows no metastases. The most likely mechanism is:

During a massive transfusion, total calcium is 2.20 mmol/L (8.8 mg/dL; reference 2.15-2.55) but ionized calcium, measured with balanced heparin, is 0.86 mmol/L (reference 1.15-1.32). Why are they so different?

Total calcium is 7.6 mg/dL and albumin is 2.0 g/dL. What is the albumin-corrected calcium (using 4.0 g/dL as normal albumin)?