Don't rely on urine specific gravity measurement when a dog has received hetastarch
Urine specific gravity (USG) and urine osmolality (UOsm) typically have a linear relationship; as USG rises, so does UOsm. However, some high-molecular-weight molecules such as albumin and iohexol have been shown to increase USG measurements in comparison to UOsm, thus altering the measurement relationship and possibly leading to an overestimation of urine concentration. A recent study in the Journal of Veterinary Internal Medicine examined whether another high-molecular-weight molecule, the colloid hetastarch, has a similar effect on USG and osmolality measurements. Eight dogs were given hetastarch, and four of these same dogs also served as the control group, with a washout period of four weeks between the treatment and control experiments. The dogs were monitored for a 360-minute period during which their urine volume, USG, and UOsm were measured every 30 minutes. The first 90 minutes were used to obtain baseline data. After 90 minutes, the treatment group received 20 ml/kg hetastarch and the control group received 20 ml/kg sodium chloride solution—both given intravenously by infusion pump over one hour.
No difference was seen in the UOsm in the treatment vs. control groups. But the USG in the dogs that received hetastarch did increase, starting at 120 minutes into the monitoring period (30 minutes after hetastarch administration). The USG appeared to peak at 150 minutes, so most of the effect was seen in the first two hours. The authors concluded that USG measurement may overestimate the urine concentration in dogs receiving or that have recently received hetastarch, possibly resulting in inappropriate therapy. For example, a patient with azotemia and an increased USG may be assessed with a prerenal cause of the abnormalities rather than recognizing that recent hetastarch therapy may be the cause. Thus, dogs that are receiving or that have recently received hetastarch should instead be assessed by UOsm since this test is unaffected by high-molecular-weight molecules.
Smart L, Hopper J, Aldrich J, et al. The effect of hetastarch (670/0.75) on urine specific gravity and osmolality in the dog. J Vet Intern Med 2009;23(2):388-391.
Link to abstract: http://www3.interscience.wiley.com/journal/121677211/abstract