Are the increasing clinical demands for osmolality measurements and their associated electrolytes appropriate?

  • W. P. Tormey


An audit of urine and plasma osmolalities and their associated urea and electrolytes over a 4 week period found that there were 124 plasma and 96 urine osmolality requests from 67 patients. In 21 patients (31.3 per cent), the osmolality results were useful in reaching a more precise diagnosis. In a further 11 cases, urine osmolality rather than plasma would have been appropriate. Seventy-one per cent originated from the Intensive Therapy Unit and were largely requested reflexly by the hospital computer order communication system. Plasma osmolal gaps could be calculated on 80 occasions (65 per cent). The formula 1.89 Na + 1.38 K + 1.03 urea + 1.08 glucose + 7.45 proved to be more accurate than the formula [Na+K] × 2 + urea + glucose (in mmol/L) with the latter showing a positive bias when compared to measured values. The osmolal gap was > 10 mOsm/Kg using the more complex formula on 23 occasions in 16 patients but only twice using the simpler calculation. These 16 patients usually had organ failure and were very ill.

Urine sodium and potassium were measured on 72 occasions in 27 of these patients but urine chloride was never requested. Urine sodium < 20 mmol/L was found in 7 patients all of whom had relative or absolute hypovolaemia. Urine sodium was measured in 73 per cent of patients investigated for SIADH in general wards. Data was available to calculate the urine osmolal gap on 52 occasions. The value was >100 mmol/L in 10 cases and this may be used as an index of the renal ammonium response to acidosis. Much potential derived information from simple indices is unused. As a result of this study, there was an approximate halving of the subsequent request volume.


Diabetes Insipidus Urine Sodium Plasma Osmolality Urine Osmolality Serum Osmolality 


  1. 1.
    Gennari, F. J. Serum osmolality: uses and limitations. N. Engl. J. Med. 1984; 310: 102–5.PubMedGoogle Scholar
  2. 2.
    Penney, M. D., Walters, G. Are osmolality measurements clinically useful? Ann. Glin. Biochem. 1987; 24: 566–71.Google Scholar
  3. 3.
    Loeb, J. N. The hyperosmolar state. N. Engl. J. Med. 1974; 290: 1184–7.PubMedGoogle Scholar
  4. 4.
    Bhagat, C. L., Garcia-Webb, P., Fletcher, E., Beilby, J. P. Calculated vs measured plasma osmolalities revisited. Clin. Chem. 1984; 30: 1703–5.PubMedGoogle Scholar
  5. 5.
    Hilborne, L. H. Howanitz, P. J., Howanitz, J. H. Serum osmolality. N. Engl. J. Med. 1984; 310: 1608.Google Scholar
  6. 6.
    Tormey, W. P. Reporting the “osmolal gap” to ‘Accident and Emergency’. Irish Med. J. 1992; 85: 159–60.Google Scholar
  7. 7.
    Tormey, W. P. Limitations of the osmolal gap with toxin ingestion. Am. J. Clin. Path. 1993; 100: 85.PubMedGoogle Scholar
  8. 8.
    Rose, B.D. Hypoosmolal states-hyponatremia. In: Clinical physiology of acid-base and electrolyte disorders. 4th ed, New York: McGraw-Hill, 1994: 651–94.Google Scholar
  9. 9.
    Steiner, R. W. Interpreting the fractional excretion of sodium. Am. J. Med. 1984; 77: 699–702.PubMedCrossRefGoogle Scholar
  10. 10.
    Lolin, Y., Jackowski, A. Hyponatraemia in neurosurgical patients: diagnosis using derived parameters of sodium and water homeostasis. Br. J. Neurosurg. 1992; 6: 457–66.PubMedCrossRefGoogle Scholar
  11. 11.
    Wallach, J. Interpretation of diagnostic tests. A synopsis of laboratory medicine. Hyposmolality, 5th edn, London: Little Brown, 1992; 42–3.Google Scholar
  12. 12.
    Eastham, R. D. Biochemical values in clinical medicine. Osmolal gap, 7th edn. Bristol: Wright, 1985; 253–4.Google Scholar
  13. 13.
    Verbalis, J. G., Gullans, S. R. Decreased brain concentrations of multiple organic osmolytes accompanies volume regulatory electrolyte losses during chronic hyponatremia. (abstr). J. Am. Soc. Nephrol. 1990; 1: 706.Google Scholar
  14. 14.
    Southgate, H. J., Burke, B. J., Walters, G. Body space measurements in the hyponatraemia of carcinoma of the bronchus: evidence for the chronic ‘sick cell’ syndrome. Ann. Clin. Biochem. 1992; 29: 90–95.PubMedGoogle Scholar
  15. 15.
    Lee, J. H., Arcinue, E., Ross, B. D. Organic osmolytes in the brain of an infant with hypernatremia. N. Engl. J. Med. 1994; 331: 439–42.PubMedCrossRefGoogle Scholar
  16. 16.
    Harrington, J. T., Cohen, J. J. Acute oliguria. N. Engl. J. Med. 1975; 292: 89–91.PubMedGoogle Scholar
  17. 17.
    Miller, T. R., Anderson, R. J., Linas, S. L., Henrich, W. L., Berns, A. S., Gabow, P. A., Schrier, R. W. Urinary diagnostic indices in acute renal failure: a prospective study. Ann. Intern. Med. 1978; 89: 47–50.PubMedGoogle Scholar
  18. 18.
    Kamel, K. S., Ethier, J. H., Richardson, R. M. A., Bear, R. A., Halperin, M. L. Urine electrolytes and osmolality: when and how to use them. Am. J. Nephrol. 1990; 10: 89–102.PubMedCrossRefGoogle Scholar
  19. 19.
    Flear, C. T. G., Gill, G. V., Burn, J. Hyponatraemia: mechanisms and management. Lancet 1981; ii: 26–31.CrossRefGoogle Scholar
  20. 20.
    Boyd, D. R., Folk, F. A., Condon, R. E. et al. Predictive value of serum osmolality in shock following major trauma. Surg. Forum 1970; 21: 32–3.PubMedGoogle Scholar
  21. 21.
    Gill, G. V., Baylis, P. H., Flear, C. T. G., Lawson, J. Y. Changes in plasma solutes after food. J. R. Soc. Med. 1985; 78: 1009–13.PubMedGoogle Scholar

Copyright information

© Springer 1997

Authors and Affiliations

  • W. P. Tormey
    • 1
  1. 1.Department of Chemical PathologyBeaumont HospitalDublin 9

Personalised recommendations