Management of electrolyte disorders: also the method matters!

Introduction: For the last few decades, electrolyte determinations in plasma or serum are carried out by reliable potentiometric methods. In recent years, a marked technical evolution has taken place, where the clinical analysis of common analytes (e.g. electrolytes) is partly moving from centralised clinical core laboratories to near-patient point-of-care testing. Methods: As the measuring principle used by point-of-care testing markedly differs from the one used in core laboratories, sodium results are not always interchangeable in critically ill patients due to the different sensitivity of the analytical methods for the electrolyte exclusion effect. Results: This effect mainly occurs in patients with decreased plasma protein values. The observed differences in generated test results might significantly affect the judgment and the treatment of electrolyte disturbances. As technical solutions are not likely to occur in the near future, clinicians and laboratorians should be well aware of this growing problem. Mathematical correction of the sodium results for plasma protein concentration may resolve the problem to a certain extent. Discussion: Although electrolyte determinations are generally very reliable, analytical interferences can occur for sodium rarely, mainly due to contamination by surfactants, benzalkonium in particular. For potassium, the major problem is hemolysis. To a lesser extent, leukocyte lysis and thrombocytopenia may also interfere. For chloride determination, the selectivity of the electrodes used is not ideal. Occasionally, false positive signals can be observed in presence of interfering ions (e.g. bromide).

An Unusual Type of Kidney Stone.

A very rare case of acetylsulfapyridine nephrolithiasis is presented in a 54-year-old female patient who had been prescribed sulfasalazine (6 x 500 mg/day) because of psoriatic arthritis for the last 9 years. The patient's renal function was only slightly impaired. Reflectance infrared spectroscopy and gas chromatography-mass spectrometry allowed the identification of the chemical nature of the stone. As acetylsulfapyridine is a metabolite of sulfasalazine, administration of the latter drug was the cause of the nephrolithiasis.