Dipstick measurements of urine specific gravity are unreliable

Abstract

Aim-To evaluate the reliability of dipstick measurements of urine specific gravity (U-SG). Methods-Fresh urine specimens were tested for urine pH and osmolality (U-pH, U-Osm) by a pH meter and an osmometer, and for U-SG by three diVerent methods (refractometry, automatic readout of a dipstick (Clinitek-50), and (visual) change of colour of the dipstick). Results-The correlations between the visual U-SG dipstick measurements and U-SG determined by a refractometer and the comparison of Clinitek ® -50 dipstick U-SG measurements with U-Osm were less than optimal, showing very wide scatter of values. Only the U-SG refractometer values and U-Osm had a good linear correlation. The tested dipstick was unreliable for the bedside determination of U-SG, even after correction for U-pH, as recommended by the manufacturer. Conclusions-Among the bedside determinations, only refractometry gives reliable U-SG results. Dipstick U-SG measurements should be abandoned. (Arch Dis Child 2001;85:155-157)

Is specific gravity a good estimate of urine osmolality?

“…Our findings for the correlation of reagent strip with osmolality (R 2 5 0.67) are generally in agreement with previous studies (r: 0.61-0.83) (1,(8)(9)(10)12). It is thought that glucose does not affect the relationship between SG and osmolality (13,14) and it is controversial whether protein does so (5).…”

“…This was true both for refractometry measured manually or by an automated refractometer as in our study (1,(8)(9)(10). In our study, we found a poorer correlation (r 5 0.73).…”

“…USG was previously measured manually through a refractometer. This has been shown to correlate well with urine osmolality (1,2). The estimation of osmolality using specific gravity (SG) has been described previously as a multiple of 35,000-40,000 (3,4).…”

Unexpected random urinary protein:creatinine ratio results–limitations of the pyrocatechol violet-dye method

“…We found, however, that urine creatinine <3 mM was more effective than urine SG ≤1.010 for identifying dilute samples in which random urinary PrCr was overestimated; the inaccuracy of urine dipstick SG compared to other measures of urine concentration is well documented [10], whereas the use of creatinine to correct for variability in urine concentration is the premise of PrCr assessment [1]. The overestimation is attributed to the fact that the accuracy of pyrocatechol violet molybdate dye-binding relies on the ionic strength of the urine.…”

Clinical utility of urine specific gravity, electrical conductivity, and color as on‐farm methods for evaluating urine concentration in dairy cattle

“…To our knowledge, only 1 study had investigated the clinical performance of the urine reagent dipstick for measuring specific gravity in urine from dairy cattle; the results of that study indicated that the urine reagent dipstick measured 0.014 lower than U SG‐R . Urine specific gravity was poorly correlated ( r = 0.36) with U Osm in dog urine ( r = 0.39), and human urine . The dipstick pad has 3 main ingredients: a cation exchanger, pH color indicator (bromothymol), and buffers.…”