Corpus: Calcium

Calcium, or Ca for short, is a divalent alkaline earth metal with atomic number 20 and an average atomic weight of about 40 g/mol.

Calcium is also spelt Kalzium in German, as the "C" does not usually appear as an independent consonant in German. In compound terms, such as "serum calcium", the German spelling is often preferred.

In its pure form, calcium is a light, very conductive, silvery metal. Its properties are similar to those of the heavier alkaline earth metals magnesium, strontium and barium. It is a good conductor of electricity and heat. When heated and slightly pressurised, it loses its metallic properties. They are only restored at higher pressures - then it can be used as a superconductor.

Due to its strong reactivity, calcium only occurs in chemical compounds under normal conditions, including as:

• Calcium carbonate
• Calcium oxalate
• Calcium phosphate
• calcium sulphate
• calcium acetate

The intracellular calcium concentration can be significantly increased by various messenger substances or electrical stimulation of the cell. The calcium ions (Ca²⁺) act as second messengers in many signalling cascades. Calcium-dependent cell processes include contraction, secretion, gene expression and mitosis.

As part of these processes, calcium ions address various proteins that have specific binding sites for calcium, including protein kinase C, phospholipase A2 and calmodulin. Calmodulin has a particularly important function as it passes calcium on to other proteins. It can bind a maximum of four calcium ions per molecule. In addition to the sarcoplasmic reticulum, it is a regulator of intracellular calcium concentration.

At around 1 to 1.1 kg, calcium is the most abundant mineral in the human body. It is therefore one of the bulk elements. In the organism, calcium is predominantly found in bone (approx. 99%), where it is stored as a phosphate salt in the form of hydroxyapatite (Ca₅[PO₄]₃OH). Here it is primarily responsible for the strength of the bone and has a reservoir function.

Other functions of the ionised form Ca²⁺:

Vitamin D3 is required for calcium absorption, which in turn is formed in the skin under the influence of UV rays, among other things. Another major influence on the calcium balance is parathyroid hormone, which indirectly increases calcium reabsorption from the primary urine in the kidneys, inhibits its incorporation into the bones and thus leads to an increase in serum calcium. Calcitonin counteracts this process by ensuring the incorporation of calcium into the bones.

In serum, almost 50 % of calcium is bound to proteins, the other 50 % is found as free ions. This proportion is influenced by proteins (low free calcium with a high protein content) and by the pH value of the blood. The free calcium in the serum increases in acidosis and decreases in alkalosis. A high serum calcium level can favour acute pancreatitis, hypertrophic osteopathy, nephrolithiasis and gastric ulcers.

See also: Hyperparathyroidism

Calcium must be supplied in sufficient quantities with food. The requirement is based on bone metabolism and is between 800 and 1,200 mg per day in adults. In adolescents, during pregnancy and breastfeeding as well as postmenopausal women, 1,500 mg and more per day are required. Foods rich in calcium include dairy products, soya, certain vegetables and mineral waters containing calcium.

Calcium is an important laboratory parameter that provides information about bone metabolism, kidney function and endocrinological relationships (parathyroid hormone, calcitriol), among other things. The calcium concentration in the blood and urine can be determined in the laboratory. Serum or heparinised blood plasma is used as a sample. The calcium concentration is given as serum calcium. Similar to the determination of serum calcium, prolonged venous congestion prior to blood collection can lead to falsely elevated values.

Serum calcium[Bearbeiten]

About 50 % of calcium in the blood is in the form of Ca²⁺ ions, 35 % is bound to proteins (especially albumin) and 15 % is complexed as bicarbonate, lactate, citrate or phosphate. When determining the calcium level in the blood, two different values can therefore be recorded: Firstly, the free, ionised calcium and secondly, the total calcium, which also includes the protein-bound and complexed calcium.

Both values are to be regarded as diagnostically equivalent, provided that the pH value of the blood and the total protein or albumin are normal. If this is not the case, there will be shifts in the determination of total calcium. In the case of abnormal protein concentrations, the value can be approximately corrected using the following formulae:^[1]

$${\displaystyle {\ce {Corrected\ Ca[mmol/l]=measured\ Ca[mmol/l]-0.025*albumin[g/l]\ +1.0}}}$$

or

$${\displaystyle {\ce {Corrected\ Ca[mmol/l]=measured\ Ca[mmol/l]/0.6+totalprotein[g/l]/194}}}$$

Due to the rapid changes in the sample tube (pre-analysis), the measurement of free, ionised calcium is more suitable for the POCT range, for example as part of a blood gas analysis.

Referenzbereich[Bearbeiten]

• Total calcium: 8.4 to 10.5 mg/dl (2.2 to 2.6 mmol/l)
• Ionised calcium: 4.5 to 5.6 mg/dl (1.1 to 1.4 mmol/l)

The reference value specified by the laboratory is decisive.

Conversion[Bearbeiten]

Measurement method[Bearbeiten]

Total calcium in serum is determined using absorption spectrometry or atomic emission spectrometry. Ionised calcium is measured using ion-selective electrodes.

Interpretation[Bearbeiten]

If the calcium level is within the physiological range, this is referred to as normocalcaemia. An increased calcium level, on the other hand, is referred to as hypercalcaemia and a decreased level as hypocalcaemia. The possible causes can be found under the respective term.

Calcium in the urine[Bearbeiten]

Calcium excretion in the urine (calcium in urine, abbreviated Ca-U) is determined using 24-hour urine collection. The urine must be acidified with hydrochloric acid to prevent calcium precipitation.

Referenzbereich[Bearbeiten]

• Men: < 7.5 mmol/day
• Women: < 6.2 mmol/day

Interpretation[Bearbeiten]

Increased calcium excretion in the urine is referred to as hypercalciuria. It can be triggered by prerenal causes, which are largely identical to those of hypercalcaemia, or by increased renal calcium excretion, e.g. in the context of renal tubular acidosis.