![]() We also hypothesized that pathological urine samples containing hemoglobin, leukocyte esterase, bilirubin, protein, nitrates, glucose, urobilinogen, and ketones would have poorer correlation between SG and osmolality whether measured by refractometry or by reagent strip as compared with “clean” urine samples not containing these constituents. We hypothesized that USG measurement using reagent strips would correlate better with urine osmolality at neutral pH than at an acidic or alkaline pH, whereas USG obtained through refractometry would correlate similarly at both acidic and neutral pH. Our institution uses both methods to measure SG, giving us a unique opportunity to compare the correlation of SG obtained by each of these methods with concurrently measured urine osmolality. Today, USG is measured mostly through automated machines which may either use refractometry or reagent strip testing. USG has not been shown to correlate well with osmolality when urine is acidic in reagent strip testing 6, 7, whereas there is no effect of pH on refractometry 5. However, in clinical practice, urine pH is often acidic, especially in the setting of acute kidney injury. Early descriptions of colorimetric assays indicated that SG was most accurate at a neutral pH of 7.0–7.5 5. However, this relationship was determined before the introduction of urine reagent strips which is a colorimetric assay (utilizing color changes). The estimation of osmolality using specific gravity (SG) has been described previously as a multiple of 35,000–40,000 3, 4. This has been shown to correlate well with urine osmolality 1, 2. USG was previously measured manually through a refractometer. Urine specific gravity (USG) is often used by clinicians in routine practice as an estimate of urine osmolality. Conclusion: In pathological urines, direct measurement of urine osmolality should be used. Pathological urines had significantly poorer correlation between USG and osmolality than “clean” urines. For an increase in SG of 0.010, predicted osmolality increases by 182 mosm/kg/H 2O for the reagent strip and 203 mosm/kg/H 2O for refractometry. At a pH of 7 and with an USG of 1.010 predicted osmolality is approximately 300 mosm/kg/H 2O for either method. The variables affecting the correlation included pH, ketones, bilirubin, urobilinogen, glucose, and protein for the reagent strip and ketones, bilirubin, and hemoglobin for the refractometry method. Results: This study demonstrated that USG obtained by both reagent strip and refractometry had a correlation of approximately 0.75 with urine osmolality. The relationships were analyzed by linear regression. The other variables considered were pH, protein, glucose, ketones, nitrates, bilirubin, urobilinogen, hemoglobin, and leukocyte esterase. Urinalysis data on these subjects were used to determine the correlation between USG and osmolality, adjusting for other variables that may impact the relationship. Out of these, 253 USG's were measured by automated refractometry and 251 USG's were measured by reagent strip. Design: Using our laboratory's records, we retrospectively gathered data on 504 urine specimens on patients on whom a simultaneously drawn USG and an osmolality were available. Objective: We studied the correlation of USG obtained by either method with a concurrently obtained osmolality. USG is measured either by refractometry or by reagent strip. Context: Urine specific gravity (USG) is often used by clinicians to estimate urine osmolality.
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