The Truth Lies Within the Gap

We report a case in which a post-analytical transcription error in serum osmolality led to diagnostic delay and unnecessary investigations in a patient with hyponatremia. The principal learning point of this case is that discrepancy between measured and calculated osmolality-the osmolal gap-was critical to detecting the error and redirecting the diagnostic evaluation. A 68-year-old female with a past medical history of hyperlipidemia, hypertension and type 2 diabetes mellitus, was scheduled for inpatient open reduction and internal fixation of a right distal radius fracture under the Hand surgery service. She was referred to the endocrinologists in view of persistent hyponatremia. Pre-operatively, her serum sodium was 128mmol/L (reference interval 135-145mmol/L), potassium was 4.2mmol/L (3.5-5mmol/L), urea was 4.8mmol/L (2.0-6.5mmol/L), random glucose was 5.2mmol/L (4.0-7.8mmol/L) and measured osmolality was 296 mOsm/Kg (275-300 mOsm/Kg). Records of her previous serum sodium trend were reviewed, and all were within the normal reference interval since 2011. In this case, the serum osmolality was within our laboratory's normal reference interval albeit at the higher end at 296 mOsm/Kg. The initial clinical impression at this point of time was thus normo-osmolar pseudohyponatremia. The clinical pathologist's opinion was requested, and the patient's serum sodium was further evaluated on the RAPIDLab® 1200 Systems Siemens blood gas analyser. This returned a similar sodium result of 128 mmol/L, excluding the possibility of electrolyte exclusion effect on the initial indirect ISE (Beckman Coulter, CA, USA). Upon further investigation, it showed that the osmolal gap was spuriously elevated (30mOsm/kg) due to a transcription error for the measured osmolality, hence confounding the true aetiology of hyponatremia. This case has shown that prompt communication between laboratory professionals and physicians will assist in resolving clinically discordant results. The osmolal gap, in particular, served as a useful adjunct in working up an ambiguous cause of hyponatremia.