Introduction

There is a close relationship of the USG and the gold standard of concentration, urine osmolality (UOsm). Hydroxyethyl starches (HES) are used to augment the colloid osmotic pressure in clinical patients. The filtered metabolites of HES increase the USG without a concomitant increase in UOsm in experimental animals.

Objectives

We hypothesized that the relationship of USG to urine osmolality would be altered in clinical patients receiving two HES formulations.

Methods

Free-catch urine samples were collected on hospitalized patients and the USG and UOsm were measured. Enrolled were 24 dogs not on IV fluids, 47 on crystalloid fluids, and 31 that were administered HES. Of those on HES, 20 of the patients were on HES 600/0.75 and 11 of the patients were administered HES 130/0.4. A refractometer measured USG while a freezing point osmometer measured the osmolality. Kruskal-Wallis test was used with a p value of < 0.05 considered significant.

Results

These data demonstrate that the ratio of UOsm to USG is decreased when patients are administered HES. The median osmolality divided by the last two digits of urine specific gravity was 34.40 (IQR 31.34–37.22) for the no fluids group, 36.03 (IQR 33.33–37.86) for the crystalloid group, and 29.25 (IQR 24.97–32.25) for the patients receiving HES. The hetastarch ratio was statistically different than the other two groups (p < 0.0001) while the others were statistically the same.

Conclusions

USG overestimates UOsm and urine concentration when HES 600/0.75 or 130/0.4 is administered to clinical patients.