Post-collection acidification of spot urine sample is not needed before measurement of electrolytes

Ascorbic acid and glucose can cause significant interference on quantitative measurement of biochemistry analytes in urine

BACKGROUND AND AIMS

Preanalytical errors due to interferences can lead to inaccurate results, necessitating an understanding of potential interferences for each test. This study explores the impact of elevated concentrations of ascorbic acid and glucose on urine analysis, a pivotal diagnostic tool.

METHODS

Conducted at the Clinical Institute of Chemistry, KBC Sestre milosrdnice, the research utilized a 24-hour urine sample. Parameters assessed included total proteins, albumin, amylase, sodium, potassium, chlorides, calcium, phosphates, magnesium, creatinine, urea, and uric acid. Various concentrations of added interferents were prepared for duplicate measurements using statistical analysis in Microsoft Excel.

RESULTS

No statistically significant interferences were found in albumin, amylase, sodium, potassium, or phosphate concentrations. However, ascorbic acid interfered with chloride, calcium, and magnesium determinations. Conversely, elevated glucose affected total protein, calcium, magnesium, creatinine, urea, and uric acid determinations. Interference of ascorbic acid with chloride and interference of glucose with total proteins and uric acid displayed a linear relationship.

CONCLUSIONS

Results suggest cautious analysis interpretation from certain parameters in patients with elevated glucose and/or ascorbic acid in urine. Whereas ascorbic acid interference may go unnoticed due to its infrequent measurement, routine determination of glucose in urine is crucial, especially for diabetes patients.

Acidification is required for calcium and magnesium concentration measurements in equine urine

BACKGROUND

Acidification of equine urine to promote dissociation of ion complexes is a common practice for urine ion concentration measurements. The objective of this study was to evaluate the effect of acidification and storage after acidification on calcium (Ca), magnesium (Mg) and phosphate (P) concentrations and on fractional excretion (FE) of these electrolytes. Thirty-two fresh equine urine samples were analysed between December 2016 and July 2020. Complete urinalysis (stick and sediment) was performed on all samples. Ca, Mg, P and creatinine concentrations were measured in supernatant of centrifuged native urine, urine directly centrifuged after acidification and urine centrifuged 1 hour after acidification. Urine was acidified with hydrochloric acid to reach a pH of 1-2. Ca, Mg, P and creatinine concentrations were also measured in blood plasma, and fractional excretion of each electrolyte was calculated. Equality of medians was tested with Friedman tests and Bland-Altman bias plots were used to show the agreement between conditions.

RESULTS

Acidification had a statistically significant effect on Ca and Mg concentrations, FECa and FEMg. Bland-Altman plot revealed a strong positive proportional bias between Ca concentration in native and acidified urine with a mean bias of 17.6 mmol/l. For Mg concentration, the difference between native and acidified urine was small with a mean bias of 1.8 mmol/l. The increase in FECa was clinically relevant. Storage of acidified urine had no effect on any of the measured ion concentrations. All P concentrations in native urine samples were below the detection limit of the assay and statistical analysis and calculation of FEP was not possible.

CONCLUSIONS

Urine acidification is essential for accurate measurement of Ca and Mg concentrations and therefore FE calculations in equine urine. Storage time of 1 hour after acidification does not significantly change Ca and Mg concentrations.