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Practice Cases in primary care laboratory medicine

Hyponatraemia and hypernatraemia: pitfalls in testing

BMJ 2007; 334 doi: (Published 01 March 2007) Cite this as: BMJ 2007;334:473
  1. W Stuart A Smellie, consultant1,
  2. A Heald, locum consultant2
  1. 1Clinical Laboratory, General Hospital, Bishop Auckland DL14 6AD
  2. 2Department of Medicine, General Hospital
  1. Correspondence to: W S A Smellie info{at}
  • Accepted 2 January 2007

Disorders of salt and water balance are extremely common in primary care. In many cases the cause is apparent and the result is not life threatening, but doctors should be aware of warning signs that may point to serious progressive disorders so that these can be diagnosed and managed early

Many situations involving the use and interpretation of laboratory tests are not supported by the high levels of evidence that can be achieved when interventions are assessed, but considerable consensus guidance is available on optimal use of laboratory tests. This article considers two scenarios involving salt and water balance that may be seen in primary care and discusses when further investigations may be helpful, and it gives a summary of evidence based and consensus guidance.

Summary points

  • Hyponatraemia is common in primary care; hypernatraemia is rarer

  • In both conditions, the common causes are usually clinically apparent

  • When great or rapid changes occur, consider rarer causes

  • Urine spot sodium concentration and osmolality help to differentiate the cause

  • Unexpected results should raise suspicion of pseudohyponatraemia and pseudohypernatraemia

Although disorders of salt and water balance are extremely common in primary care, their causes are usually apparent, and the primary clinical question that arises is whether any change of dosage or drugs is required (typically diuretics in heart failure).

Serious and rapidly progressing sodium and water balance problems are rarer, and the practitioner often needs to decide when to initiate further investigation.

Case 1

A 72 year old man with chronic obstructive pulmonary disease due to longstanding smoking presented to his general practitioner with a two week history of lethargy and feeling nauseated. The patient was being treated with furosemide 20 mg and enalapril 5 mg a day for congestive cardiac failure (doses unchanged over recent months). This was clinically stable, and he had been noted previously to …

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