[Comparative measurement of urine specific gravity: reagent strips, refractometry and hydrometry].
BACKGROUND
The urine specific gravity is commonly used in clinical practice to measure the renal concentration/dilution ability. Measurement can be performed by three methods: hydrometry, refractometry and reagent strips.


OBJECTIVE
To assess the accuracy of different methods to measure urine specific gravity.


METHODS
We analyzed 156 consecutive urine samples of pediatric patients during April and May 2007. Urine specific gravity was measured by hydrometry (UD), refractometry (RE) and reagent strips (TR), simultaneously. Urine osmolarity was considered as the gold standard and was measured by freezing point depression. Correlation between different methods was calculated by simple linear regression.


RESULTS
A positive and acceptable correlation was found with osmolarity for the RE as for the UD (r= 0.81 and r= 0.86, respectively). The reagent strips presented low correlation (r= 0.46). Also, we found good correlation between measurements obtained by UD and RE (r= 0.89). Measurements obtained by TR, however, had bad correlation when compared to UD (r= 0.46). Higher values of specific gravity were observed when measured with RE with respect to UD.


CONCLUSIONS
Reagent strips are not reliable for measuring urine specific gravity and should not be used as an usual test. However, hydrometry and refractometry are acceptable alternatives for measuring urine specific gravity, as long as the same method is used for follow-up.
Costa
Christian Elías
CE
Unidad de Nefrología, Hospital de Niños Ricardo Gutiérrez. c.eliascosta@gmail.com
Bettendorff
Carolina
C
Bupo
Sol
S
Ayuso
Sandra
S
Vallejo
Graciela
G
D002648:Child; D003430:Cross-Sectional Studies; D006801:Humans; D011446:Prospective Studies; D011934:Reagent Strips; D012031:Refractometry; D013046:Specific Gravity; D016482:Urinalysis / Q000379:methods*; D014556:Urine
D011934:Reagent Strips