Ryane E. Englar Urine specific gravity (USG) is measured during routine urinalysis as a test of how well the kidneys can modify urine concentration by adjusting its solute content [1]. When we measure the specific gravity (SG) of a solution, we are determining that solution’s density [2]. Density is a measure of the mass of an object relative to the space that it occupies [2]. When we consider density of a solution, then what we are really investigating is the mass of solute per volume of solution. Urine is a mixture of solutes. Therefore, urine’s density is a measure of the mass of all solutes within a given volume of solution. To interpret this, we compare the density of urine to the density of an equal volume of distilled water [3]. This ratio is essentially what defines USG [1–3]. Fresh water has a SG of 1.000 at 4° Celsius at sea level [2]. Substances that are less dense than water will float. These include gasoline, automotive oil, kerosene, jet fuel, lard oil, and corn oil. These substances have a SG less than 1.000. Substances that are denser than water will sink. These include milk, 5% sodium chloride, and propylene glycol. These substances have a SG greater than 1.000. Urine is always denser than water because it is excreted by the kidneys and based upon the physiologic process by which urine is produced, the kidneys are incapable of excreting pure water. Urine always consists of water plus solutes of varying densities. In addition to water, urine contains: Therefore, USG will always exceed 1.000. How much of each solute is contained within a given volume of urine determines the degree to which USG exceeds 1.000. For example, adding one of the following substances to 100 ml of urine will increase USG by 0.001 [2]: What determines how much solute is excreted by the kidneys depends upon the patient’s hydration status and renal function. The kidneys must be able to sense and respond appropriately to the tonicity of the patient’s plasma. Response depends upon renal tubular function, which determines the degree to which urine will be diluted within the loop of Henle or the degree to which urine will be concentrated within the distal tubules. In addition to renal function, how much urine is produced and how concentrated that urine will be depends upon several factors: [4, 5] In health, urination, and thirst are interrelated. Assuming adequate renal function, a dehydrated patient will both maximally concentrate urine to conserve water and be driven to drink [5]. Urine formation requires a balancing act by the kidneys [5]. If the body has too little water, then the kidneys step in to conserve water and urine output will decline [5]. On the other hand, if the body is experiencing water overload, then the kidneys will eliminate the excess as urine [5]. In this way, the kidneys work in concert with the rest of the body to achieve water balance. Water balance is made possible by the following factors working in concert: [4–12] ADH is produced by the hypothalamus [4, 6]. ADH is secreted when plasma tonicity increases [4, 6, 11]. ADH conserves water by reabsorbing it from the distal tubules and collecting ducts of the nephrons through aquaporin channels [4, 6, 13]. This action reduces urine output and results in concentrated urine [4, 6]. Urine cannot be adequately concentrated unless at least one‐third of nephrons are operational [6]. Renal medullary hypertonicity is also required for water to be passively reabsorbed in the distal tubule and collecting duct [6]. This concentration gradient is established and maintained by the movement of sodium, chloride, and urea out of the nephrons and into the medullary interstitial space [14]. Measuring USG is an easy, cheap, and convenient way to gain information about a patient’s hydration status and their renal tubular function. As clinicians, we want to know how well the kidneys are regulating water balance and excreting waste. USG is our attempt to describe urine concentration (see Figure 13.1). The gold standard approach to estimating urine concentration is to measure urine osmolality. Osmolality describes the number of particles in a solution irrespective of particle size or weight, a feature that makes it more accurate [15]. For example, USG overestimates urine solute concentration if many high molecular weight molecules (e.g., albumin, synthetic colloids, iohexol) are present in the urine whereas urine osmolality does not [15, 16]. Despite its accuracy, osmolality is not easily measured in clinical practice as compared with USG. Because USG can be performed in‐house with ease, it has become a standard part of routine urinalysis. Figure 13.1 Three different samples of urine with varying degrees of concentration. The sample on the far left is hypersthenuric. The sample on the far right is hyposthenuric. The sample in the middle is isosthenuric. We cannot determine this without measuring USG. However, the color of the urine on the far left suggests that the sample is highly concentrated as opposed to the sample on the far right, which appears to be very dilute. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. USG and osmolality are linearly correlated; thus, it is appropriate to extrapolate urine concentration from USG [15]. Four canine studies have reported excellent correlations between USGs determined by refractometry and urine osmolality measurements [17]. USG provides baseline data as well as an opportunity for repeat measurements, which are essential when monitoring patients with known or suspected alterations in fluid volume status, renal dysfunction, and other metabolic conditions, including, but not limited to, diabetes insipidus. The preferred method of in‐house measurement of USG requires refractometry [1, 2, 18]. Traditionally, refractometers are handheld tools that indirectly measure USG by detecting how solutes in solution refract light as compared with light refraction in air [1]. This so‐called refractive index (RI) depends upon the sample’s temperature and concentration [1]. Because cells, casts, and most crystals do not refract light, they do not influence the RI of urine; however, electrolytes, protein, glucose, urea, and creatinine do [1]. So, too, do particles that are suspended in solution; therefore, samples that are turbid should be centrifuged first such that only the supernatant is used to measure USG [1]. Traditional handheld refractometers contain the following components: Note that there are many types of handheld refractometers. Veterinary‐specific models have historically been promoted over those with human‐based USG scales out of concern that the latter led to spuriously high results, particularly for highly concentrated feline samples [1]. Because of this, conversion tables were created to adjust feline USG values when human health‐care refractometers were used to perform veterinary urinalysis. Feline USG can be calculated as follows [1, 19]: Veterinary‐specific refractometers are commercially available and most incorporate different USG scales for canine and feline patients. However, it is unclear whether feline‐specific scales are truly necessary [20, 21]. The accuracy of some feline‐specific refractometers has been called into question [20, 21]. It is important to consider that refractometers are likely to vary in terms of their results; therefore, using the same type of refractometer in clinical practice is important when making comparisons between patient samples. If, for instance, your patient has USG measured every month, then it would be important to use the same tool each time to evaluate patient data for trends. Regardless of whether it is manufactured for human health care or veterinary medicine, the in‐clinic refractometer should be temperature‐compensated. This feature adjusts for temperatures up to 100° Fahrenheit, which affects the density of urine [1]. If the clinic only has access to nontemperature‐compensated refractometers, then samples should be read at or near 20° Celsius, which is roughly room temperature or 68° Fahrenheit [22]. In addition to access to a refractometer, you will need the following to measure USG in‐house: Digital refractometers are now commercially available. These are easier to read and eliminate the subjectivity of interpretation. However, most clinics continue to use traditional handheld models to measure USG. The following instructions assume that you will be using a traditional refractometer. Skip to Step #4 if your UG or SG scale did in fact read 1.000. To calibrate your refractometer: Figure 13.2 Supplies for USG by Refractometry. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 13.3 Refractometer prior to calibration. The stage cover is closed. Ryane E.Englar (Author). Figure 13.4 Refractometer ready for calibration. The stage cover has been lifted. Source: Ryane E.Englar (Author). Figure 13.5 Applying a drop of distilled water to the stage for the purpose of calibration. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 13.6 Distilled water has been placed on the stage for the purpose of calibration. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 13.7 The correct positioning of the examiner’s eye relative to the refractometer to obtain a reading. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine.
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Urine Specific Gravity
13.1 Procedural Definition: What Is This Test About?
13.2 Procedural Purpose: Why Should I Perform this Test?
13.3 Equipment
13.4 Procedural Steps
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