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Sodium – Na+
Reference interval Na+ – examples
Distribution and physiological significance of sodium
Why measure sodium ?
Terms used in interpretation of sodium
Causes of hyponatremia
Symptoms of hyponatremia
Causes of hypernatremia
Symptoms of hypernatremia
A note on pseudohypo- and pseudohypernatremia
Sodium (Na+) is the dominant cation in the extracellular fluid, where it has a 14-fold higher concentration (∼140 mmol/L) than in the intracellular fluid (∼10 mmol/L). Na+ is a major contributor of the osmolality of the extracellular fluid and its main function is largely in controlling and regulating water balance, and maintaining blood pressure. Na+ is also important for transmitting nerve impulses and activating muscle concretion.
Reference interval Na+ – examples
Distribution and physiological significance of sodium
Around 30 % of the approximately 4000 mmol (92 g) of Na+ present in the human body is in the form of complexes within bone; almost all of the remaining is in the extracellular fluid. As the most abundant extracellular fluid solute, Na+ is the major determinant of its osmolality and thereby the principal determinant of water distribution between the intracellular and extracellular compartments (Fig. 13). This highlights the role of Na+ in the maintenance of blood volume and thereby blood pressure. Dysnatremia can arise as a result of disturbance of water balance (most common), disturbance of Na+ balance (less common), or a combination of both. If extracellular fluid Na+ concentration (cNa+) is too low (hyponatremia), water moves into the cells to balance the levels, causing the cells to swell. This is particular dangerous in brain cells, as their expansion increases intracranial pressure, causing cerebral edema .
Why measure sodium?
Disturbance of sodium and water metabolism and consequent abnormality in cNa+ (called dysnatremia) is a potential feature of a number of acute and chronic illnesses, some of which are relatively common. It is also a potential adverse effect of some commonly prescribed drugs . Reduced cNa+ (hyponatremia) occurs in 15 – 20 % of hospitalized patients ; increased cNa+ (hypernatremia) is less common, affecting around 1 – 2 % of hospitalized patients . Identification of dysnatremia is important because if it remains uncorrected, it can, if sufficiently severe, cause significant morbidity and may be fatal. Dysnatremia has been shown to be an independent risk factor for death among the critically ill patients .
Measurement of cNa+ is essential for calculating the anion gap, a parameter of high diagnostic utility particularly among those with acid-base disturbance (see AG).
The body preserve cNa+ within normal limits by continuously adjusting renal loss of water (urine volume) so that extracellular fluid water content is constant, despite varying water intake . The thirst response and appropriate release of the antidiuretic hormone arginine vasopressin (AVP) is necessary for this control of extracellular fluid water content.
The preservation of normal cNa+ also depends on total body sodium balance. A minimum of 10 – 20 mmol (0.23 – 0.46 g) of Na+ is lost from the body each day in urine, sweat and feces, and this must be replaced to remain in balance. In fact, a normal diet usually contains far in excess of this minimum. Daily Na+ intake (predominantly in the form of salt flavoring) is usually around 150 – 170 mmol (3.4 – 3.9 g) but can range from less than 100 mmol (2.3 g) to more than 300 mmol (6.9 g) . Excess Na+ is excreted by the kidneys in urine, and Na+ balance depends critically on the ability of the kidneys to regulate Na+ excretion so that it matches intake. This renal process depends on the adrenal hormone aldosterone, and on an intact renin-angiotensin pathway for appropriate release of aldosterone . Na+ is a constituent of gastrointestinal secretions such as bile and pancreatic juice. In total, around 1500 mmol (34 g) of Na+ is secreted into the gastrointestinal tract every day. Normally, nearly all of this Na+ is reabsorbed and only 5 – 10 mmol (0.11 – 0.23 g) is excreted in feces, but increased loss of Na+ via the gastrointestinal tract, e.g. in vomit or diarrhea, can lead to severe Na+ imbalance (depletion).
In summary, the maintenance of Na+ within normal limits depends on:
- Diet containing a minimum amount of Na+ (at least 20 mmol (0.46 g)/day)
- Intact thirst response and free access to water
- Normal renal function
- Normal gastrointestinal function
- Appropriate release of the hormone arginine vasopressin by the pituitary gland
- Appropriate release of the hormone aldosterone by the adrenal gland
- Intact renin-angiotensin pathway
Disturbance of any of the above can cause abnormality in cNa+.
Terms used in interpretation of sodium
Decreased cNa+ (i.e. <135 mmol/L) is called hyponatremia .
Increased cNa+ (i.e. >145 mmol/L) is called hypernatremia .
Since dysnatremia can be due to a variable combination of Na+ and/or water depletion/retention, it is important that the extracellular volume (water) status of patients with hyponatremia and hypernatremia is assessed during clinical examination to one of three states:
- Hypovolemia (decreased circulating blood volume)
- Hypervolemia (increased circulating blood volume)
- Euvolemia (normal circulating blood volume)
This allows the important etiological/diagnostic distinction between the following conditions:
- Hypovolemic hyponatremia/hypernatremia
- Hypervolemic hyponatremia/hypernatremia
- Euvolemic hyponatremia/hypernatremia
Causes of hyponatremia 
- Heart failure
- Hyperglycemia – diabetic ketoacidosis (DKA)
- Acute kidney disease
- Chronic kidney disease
- Syndrome of inappropriate antidiuretic hormone (SIADH) (a potential adverse effect of many commonly prescribed drugs and a complicating feature of some malignant diseases)
- Chronic vomiting/diarrhea
- Addison’s disease (adrenal insufficiency)
- Diuretic therapy
- Fluid replacement therapy
Symptoms of hyponatremia
Mild hyponatremia (cNa+ 130 – 135 mmol/L) is usually asymptomatic, although chronic mild hyponatremia is associated with increased risk of osteoporosis and bone fractures.
Moderate hyponatremia (cNa+ 125 – 130 mmol/L) may cause :
- Abdominal cramps
Severe hyponatremia (cNa+ <125 mmol/L) can additionally cause any of the following neurological symptoms due to brain swelling (cerebral edema):
- Impaired mental function
The most severe hyponatremia (cNa+ <115 mmol/L) can result in seizures, coma and death.
The neurological effect of hyponatremia is more severe if it has developed acutely (less than 48 hours).
Causes of hypernatremia [130, 132]
- Chronic kidney disease
- Inadequate water intake (common in the elderly)
- Failure of the thirst response due to unconsciousness, head injury
- Conn’s syndrome/disease
- Cushing’s syndrome/disease
- Diabetes insipidus
- Over-vigorous sodium replacement therapy
- Lithium therapy
Symptoms of hypernatremia
Mild hypernatremia (cNa+ 145 – 150 mmol/L) is usually asymptomatic.
More severe hypernatremia may be associated with the following [130,132]:
- Muscle weakness
Severe hypernatremia (cNa+ >160 mmol/L), particularly if with acute onset, is associated with dehydration of brain cells and the following resulting neurological symptoms:
- Altered consciousness
Acute-onset severe hypernatremia is potentially fatal.
A note on pseudohypo- and pseudohypernatremia
Falsely low or high cNa+ (pseudohypo- or pseudohypernatremia) may be reported if plasma contains a particularly high or low, respectively, concentration of lipids or protein .
Both pseudohypo- and pseudohypernatremia reflect measurement artefacts that depend on the method used to determine the cNa+ .
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Effective Date January 20th, 2023 (last updated January 20th, 2023)
ABOUT THIS POLICYRadiometer values your privacy and the protection of your personal data. This policy (“Policy”) explains how Radiometer its affiliates, subsidiaries or related companies, a full list of which can be located here (together, “Radiometer”, “our”, “us”, or “we”), collects, uses, shares, transfers and processes data collected from or about you.
“Personal Data” is any information that can be used to directly or indirectly identify an individual or that can be reasonably expected to link to an individual. This can include items such as name, address, telephone number, credit card details, email address, ID number, Internet Protocol (“IP”) address of an electronic device used by an individual, or other identifying code (even absent of other identifying information). Statistical and non-identifiable metric data are not considered Personal Data.
The Radiometer subsidiary, affiliate or related company with which you interact is, where applicable, the data controller (or equivalent under applicable law) responsible for the processing of your Personal Data. You can find a list of the relevant legal entities that act as data controllers in Appendix 1 to this Policy.
SCOPEThis Policy describes the types of Personal Data that we may collect, process or disclose about you and how you may govern this processing by exercising applicable legal rights. This Policy applies to both online and offline information collection, including your use of websites or subdomains operated by us, any mobile applications, when we provide products and/or services to you or notify you about prospective items of interest and in other situations where you interact with us in-person, by telephone or by mail where this Policy is posted or referenced.
There may be occasion where you have been provided with a circumstance-specific privacy notice that is separate from this policy, such as privacy notices for specific activities such as Recruitment. To the extent you were provided with a different notice, those notices apply and govern our interactions with you. If you provide Personal Data about parties other than yourself, you are responsible for ensuring their knowledge of how we will process their personal data, and, where applicable, obtaining any necessary consents required in advance.
We are committed to processing Personal Data in accordance with applicable laws. Please note that if you do not wish to provide your Personal Data to us, some products and/or services may become unavailable to you. Your use of any or all these platforms indicates you have been notified of our collection, use, transfer, and disclosure of your information as described in this Policy to the extent permitted by applicable law.Read more