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.

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.