Hello, my name is Qing Meng. I am the Section Chief of Clinical Chemistry Laboratories and
Professor in the Department of Laboratory Medicine, The University of Texas MD Anderson Cancer Center. Welcome to this Pearl of Laboratory Medicine on “Hypercalcemia: Causes and Laboratory Investigation.”
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This talk is to:
Provide brief review on calcium homeostasis
Discuss causes of hypercalcemia
Identify laboratory approach to hypercalcemia
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Calcium is the 5th most common element and the most prevalent cation in the body. There is approximately 1 kg of calcium in human body. Of this, 99% is distributed in bones, 1% in soft tissue and 0.1% in body fluids. In blood, virtually all of the calcium is in the serum or plasma, which is approximately 8.6-10.2 mg/dL with the ionized calcium concentration 1.13-1.32 mmol/L. The intracellular concentration of calcium is 10,000 times less than that in the extracellular fluid.
Calcium exists in three physiochemical states in plasma: approximately 50% is free or ionized calcium, which is the biologically active form, approximately 40% is bound to plasma proteins, and 10% is complexed with small anions such as bicarbonate, lactate, citrate and phosphate. Of the protein-bound calcium, approximately 80% is associated with albumin and 20% associated with globulins. The calcium binding to negatively charged proteins or anions is pH-dependent.
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Calcium homeostasis is mainly regulated by parathyroid hormone or PTH and 1,25 (OH)2 vitamin D. PTH is secreted by parathyroid glands in response to low serum ionized calcium. The physiological function of calcitonin on calcium homeostasis is not significant. Pharmacological dose of calcitonin reduces blood concentration of calcium against the action of PTH.
Calcium homeostasis is mainly regulated by these three hormones through three organs: Bone, Kidneys, and Intestine. Circulating calcium is excreted by glomerular filtration and reabsorbed in the proximal tubules.
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This table summarize the functions of the three hormones on calcium homeostasis.
PTH:
Increases bone resorption to release calcium and phosphate from bones.
Increases renal distal tubule reabsorption of calcium, inhibits tubular reabsorption of phosphate, activates 1 α-hydroxylase and stimulates conversion of 25(OH)D to 1,25(OH)2D in kidneys
Increased 1,25(OH)2D enhances calcium and phosphate absorption by intestine, and thus PTH indirectly acts on intestine for calcium absorption.
PTH response is very sensitive to changes in serum ionized Ca, either being secreted or shut off as appropriate, and PTH may be the only hormone responsible for min-to-min regulation.
1,25 (OH)2 Vitamin D maintain calcium homeostasis in 3 Ways:
It increases calcium and phosphate absorption by the small intestine.
Weakly increases calcium and phosphate resorption from the skeleton.
Weakly promotes the reabsorption of calcium ions by the kidney.
While Calcitonin decreases plasma calcium and phosphates by:
Inhibiting bone resorption, stimulating incorporation of calcium and phosphate to bone, against the effect of PTH on bone resorption.
Reducing reabsorption of calcium and phosphate by kidneys
Overall, the physiological role of calcitonin in calcium control is not clear. It may only respond to a fairly large increase in ionized Ca (~10%). It is used clinically in treatment of hypercalcelmia and in certain bone diseases by suppressing bone resorption. It is more important in regulating bone remodeling than in Ca2+ homeostasis.
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There are numerous factors can cause hypercalcemia including preanalytical, analytical, biological, and pathological.
For preanalytical errors, these include:
Tourniquet use and venous occlusion: which is associated with increased-protein bound calcium or total calcium but not ionized calciumd
Fist clenching or forearm exercise that can decrease pH and increase in ionized calcium
Hyperventilation causing respiratory alkalosis reduces ionized calcium without change of total calcium
Changes in posture such as standing decreases intra-vascular water and increases protein concentration and thus total calcium
Prolonged immobilization and bed rest increase both total and ionized calcium levels
Contamination with calcium from corks, glassware, tubes, and drywall increases calcium levels
There are also analytical interferences such as hemolysis, icterus, lipemia on Spectrophotometer or ISE.
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There are a number of pathological disorders can cause hypercalcemia.
These are:
Increased parathyroid hormone (PTH) due to Primary hyperparathyroidism
Sporadic, familial, multiple endocrine neoplasia 1 (MEN 1) or MEN 2A
Secondary and Tertiary hyperparathyroidism
Coexisting malignancy and primary hyperparathyroidism
Ectopic PTH in malignancy (though very rare)
Malignancy which is characterized by:
Humoral hypercalcemia of malignancy as mediated by
Parathyroid hormone-related protein (PTHrP)
Local osteolysis in multiple myeloma
Other malignancy such as breast cancer, lung cancer
Vitamin D intoxication, Vitamin D supplements, vitamin D metabolites or analogs
Lymphoma (1,25[OH]2D)
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Renal Failure, for example:
Chronic renal failure with treatment with calcium and 1,25[OH]2D or VitD analogs
Recovery phase of Rhabdomyolysis and acute renal failure Renal transplant
Other Endocrine Diseases such as Hyperthyroidism (Thyrotoxicosis) Adrenal insufficiency, Acromegaly Pheochromocytoma
Medications include:
Thiazide diuretics
Lithium-related release of PTH
Milk–alkali syndrome (calcium and antacids)
Vitamin A intoxication
Theophyline
Other causes such as:
Immobilization with high bone turnover (e.g., Paget’s disease, bedridden child)
Familial hypocalciuric hypercalcemia
Williams Syndrome
Hyperphosphatemia
Acute hypomagnesemia
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In general:
Primary hyperparathyroidism and cancer account for 90% of cases of hypercalcemia.
For Outpatients, 90% of the cases have primary hyperparathyroidism
For Inpatients
65% of the cases have cancer
25% of the cases are due to primary hyperparathyroidism
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In Primary Hyperparathyroidism, the increased calcium levels are due to excessive PTH secretion from parathyroid adenoma (90%) or hyperplasia multiple gland enlargement (10%) including MEN 1, MEN 2A, Familial hyperparathyroidism, Carcinoma (<1%), and familial benign hypercalcemia (FBH).
The pathophysiological change of hypercalcemia is related to combined effects of excessive secretion of PTH and production of 1,25-(OH)2-vitamin D.
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Primary hyperparathyroidism (PHPT) is diagnosed based on:
Intact PTH and chemistry panel:
PTH is elevated despite elevated serum calcium
Serum phosphate is in the low-normal to mildly decreased range
Serum ALP increased, and
with hyperchloremic metabolic acidosis
Urine calcium is normal to slight increase
24-hour urine calcium excretion is
used to rule out familial hypocalciuric hypercalcemia.
Values below 100 mg/24 hours or a calcium creatinine clearance ratio of < 0.01 are suggestive of familial hypocalciuric hypercalcemia (FHH)
Whether to use ionized calcium versus serum total calcium: there is still a debate….some labs do and some labs only use total calcium as there is approximate correlation with the ionized calcium. Also some labs especially in Europe use
Corrected serum calcium
There are many equations but here is the one calculated as: Serum calcium (mg/dL) + 0.8 [4-albumin (g/dL)]
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The Hypercalcemia of Malignancy is attributed to excessive bone resorption by one of three mechanisms.
Humoral hypercalcemia of malignancy: over-expression of PTH- related protein (PTHrP) affects bone and kidney similar to PTH. Increased expression of PTH-rP is seen in
Squamous cell carcinoma: lung, head and neck, esophagus, cervix, etc.
Carcinoma: ovarian, breast, renal, etc.
Increased 1,25-(OH)2-D synthesis by lymphomas
Local osteolytic hypercalcemia in multiple myeloma, this accounts for 20% of all hypercalcemia of malignancy
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A number of laboratory tests can be used for assessment of hypercalcemia. These include:
Serum total calcium: which is valuable as a screen and monitoring ambulatory persons.
Ionized calcium: which is more accurate assessment of calcium status
Albumin: corrected calcium
PTH
25 (OH)vitamin D and 1,25 (OH)2D:
Acid-base status
Electrolytes, Magnesium, Bicarbonate Serum BUN and creatinine
Serum phosphate
Serum (urine) protein electrophoresis
24 hour urinary calcium, calcium to creatinine ratio Urine creatinine
Urinary cyclic adenosine monophosphate (cAMP): cAMP is increased in primary hyperparathyroidism
PTHrP: is ordered in suspected malignancy
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More specifically, the following Initial laboratory tests should be ordered:
Total Calcium: is reliable
You may correct calcium value if albumin decreased:
Corrected Ca = Ca (mg/dl) + 0.8 (4-albumin [g/dl])
If calcium elevated above 11.0 mg/dL – then order intact PTH along with other tests such as:
Ionized calcium
Electrolytes– including BUN and creatinine
Phosphorus
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If PTH is:
Elevated or normal – suggests hyperparathyroidism (elevated calcium should suppress PTH); then order 24- hour urine calcium
If urine calcium is High – suggestive of primary hyperparathyroidism
If Low (<50-100 mg) – then familial hypocalciuric hypercalcemia (FHH)
If PTH is Low – then order PTHrP or consider other testing for vitamin D excess, milk-alkali syndrome, and hyperthyroidism
If PTHrP is:
Low or normal – order vitamin D: 25-(OH)D and 1,25-(OH)2D
If Vitamin D level is High – suggestive of lymphoma or granulomatous disease (sarcoidosis, Wegener granulomatosis, Tuberculosis)
If Low – consider testing for cancer
If PTHrP is High – it is suggestive of cancer
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If needed, additional tests or exanimations should be performed. For example:
Chest X-ray: for lung cancer or metastases, sarcoidosis, or tuberculosis
Renal function test: for renal failure, milk-alkali syndrome, or renal impairment in primary hyperparathyroidism or myeloma
Complete blood count: in hematological malignancy or anemia
Erythrocyte sedimentation rate or CRP: may be increased in cancer
Serum alkaline phosphatase: may be increased in primary hyperparathyroidism, Paget's disease with immobilization, myeloma, or bone metastases
Liver function tests: for liver metastases; chronic liver failure is also a rare cause of hypercalcemia.
Thyroid function tests: for Thyrotoxicosis
Serum cortisol:though Addison's disease is a rare cause
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Here is the algorithm we have developed for the investigation of hypercalcemia. Due to time constraint, I won’t repeat as most of the tests and procedures we have already discussed
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In Summary:
Primary hyperparathyroidism and cancer are the most common causes of hypercalcemia
Intact PTH is the most important test in the differential diagnosis of hypercalcemia
24 hour urine calcium and calcium to creatinine ratio are helpful
PTHrP should be measured if cancer is suspected
Tests for other causes of hypercalcemia should be ordered for differential diagnosis
An investigation algorithm should be established in each institution
