Abstract: SA-PO921

Comparison of Five Methods of Measuring Specific Gravity in Mock Urine Solutions

Session Information

  • Educational Research
    November 04, 2017 | Location: Hall H, Morial Convention Center
    Abstract Time: 10:00 AM - 10:00 AM

Category: Nephrology Education

  • 1301 Educational Research

Authors

  • Saxton, Heidi, University of Utah School of Medicine, Salt Lake City, Utah, United States
  • Ramkumar, Nirupama, University of Utah, Salt Lake City, Utah, United States
  • Gregory, Martin C., None, Salt Lake City, Utah, United States
Background

Urine specific gravity (SG) is commonly used as an indicator of urine concentration in the clinical nephrology setting. SG measurements can aid in distinguishing pre-renal etiologies from acute tubular necrosis, although the accuracy of these measurements by different methods remains unknown. Our main objective in this study was to compare five methods of measuring SG in solutions that resemble normal and pathologic urine specimens.

Methods

We measured the SG and osmolality of solutions with varying concentrations of salts, glucose, urea, albumin, and intravenous contrast using the methods of hydrometry (urinometer), refractometry, reagent strips, and pycnometry. Samples were also sent to the hospital clinical laboratory for measurement by automated refractometry. Slope of SG versus osmolality and Pearson’s correlation coefficient was calculated for each method.

Results

Slopes of SG by hydrometry correlated most closely with slope of specific gravity by pycnometry across all solutions (r value of 0.996). Slopes of SG by refractometry and clinical laboratory methods correlated moderately well (r values of 0.950 and 0.927, respectively), while slopes of SG by reagent strip correlated very poorly (r value of -0.434).

Conclusion

As mock urine solutions become more concentrated, the method of hydrometry correlates most closely with pycnometry. Refractometry manually and by clinical laboratory have moderate accuracy, while the method of reagent strip performs very poorly. This latter method may have poor clinical utility, especially in pathologic urine from patients with kidney disease.

Correlation of slope of specific gravity against calculated osmolality with pycnometry slope
     
 RefractometryReagent stripHydrometryClinical laboratory
All solutions0.9499-0.43410.99630.9265
NaCl, Urea, Albumin/urea, Glucose/urea0.9701-0.16860.99840.9494