Urine Screening with the UF-Series Analysers: The Use of Urine Conductivity as a Surrogate Marker of Urine Osmolality and Renal Diuresis

• Urine Screening with the UF-Series Analysers: The Use of Urine Conductivity as a Surrogate Marker of Urine Osmolality and Renal Diuresis

AUTHOR(S)

Eva GAZINSKY and Fritz BOEGE

Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich Heine-University, Medical School

SUMMARY

The kidney is capable of altering the water content of the urine. Consequently, analytes in the urine are diluted in a varying volume, which poses a problem when they are to be expressed in quantitative terms. To what extent a urine specimen is subjected to diuresis/antidiuresis, can best be determined by its osmolality. However, direct osmolality measurements are not suitable for routine clinical screening, which usually employs surrogate parameters, e.g. relative density or creatinine concentration of the urine. These markers are not practical, when urine screening is performed with a Sysmex UF-Series analyser as the sole device. However, such analysers measure the electrical conductivity of each urine specimen for internal reference purposes and we propose here that conductivity can serve as a robust marker of diuresis/antidiuresis. It correlates precisely with the urinary concentration of sodium and potassium, whereas it is insensitive to urea and glucose. Thus, conductivity faithfully monitors urinary electrolyte concentration, but is not influenced by pathological conditions altering urine osmolality in a fashion unrelated to diuresis. It should also be noted that conductivity correlates well with urinary creatinine concentration, a surrogate parameter of diuresis used for reference purposes. In summary, conductivity is a superior surrogate parameter for judging the state of diuresis in routine urine screening and is a suitable reference value for quantitative determinations of other urine analytes. When it comes to judging the osmotic labour of the kidney in the presence of increased concentrations of non-ionic osmolytes such as glucose or urea, conductivity complements measurements of osmolality.

KEY WORDS

Urinalysis, Osmolality, Conductivity, Relative Density, UF-100