Athletic Training and Sports Health Care

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Original Research 

Comparison of Refractometry, Urine Color, and Urine Reagent Strips to Urine Osmolality for Measurement of Urinary Concentration

Lindsey E. Eberman, PhD, ATC, LAT; Dawn M. Minton, MS, ATC, SCAT; Michelle A. Cleary, PhD, ATC

Abstract

This study sought to validate clinical measures of urine concentration against the criterion standard, urine osmolality. During preparticipation physical examinations, athletic trainers identified collegiate football players and monitored hydration status (n = 69 samples) during preseason practices. Participants were Division I collegiate American football players (n = 5; mean age, 20.4±0.5 years; mean mass = 118.7±17.8 kg; mean height = 183.6±5.8 cm). Urine samples were analyzed for urine specific gravity using refractometry, urine color, 2 brands of urine reagent strips, and urine osmolality. The relationship of the variables was assessed using Pearson and Spearman’s rho correlations. Results showed a strong correlation between urine osmolality and clinical refractometry (Pearson, r = .872), a marked correlation between urine osmolality and both urine reagent strips (Spearman’s rho, r = .647 and .626), and a moderate correlation between urine osmolality and urine color (Spearman’s rho, r = .540). With other measures, clinical refractometry is a valid field measure of urine concentration.

Abstract

This study sought to validate clinical measures of urine concentration against the criterion standard, urine osmolality. During preparticipation physical examinations, athletic trainers identified collegiate football players and monitored hydration status (n = 69 samples) during preseason practices. Participants were Division I collegiate American football players (n = 5; mean age, 20.4±0.5 years; mean mass = 118.7±17.8 kg; mean height = 183.6±5.8 cm). Urine samples were analyzed for urine specific gravity using refractometry, urine color, 2 brands of urine reagent strips, and urine osmolality. The relationship of the variables was assessed using Pearson and Spearman’s rho correlations. Results showed a strong correlation between urine osmolality and clinical refractometry (Pearson, r = .872), a marked correlation between urine osmolality and both urine reagent strips (Spearman’s rho, r = .647 and .626), and a moderate correlation between urine osmolality and urine color (Spearman’s rho, r = .540). With other measures, clinical refractometry is a valid field measure of urine concentration.

Dr Eberman is from the Department of Athletic Training, Indiana State University, Terre Haute, Ind; Ms Minton is from the Department of Physical Education, University of South Carolina, Columbia, SC; and Dr Cleary is from the Department of Kinesiology and Rehabilitation, University of Hawaii at Manoa, Honolulu, Hawaii.

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Lindsey E. Eberman PhD, ATC, LAT, Department of Athletic Training, Indiana State University, 401 North 4th Street, Arena C-10, Terre Haute, IN 47809; e-mail: leberman@indstate.edu.

Received: February 16, 2009
Accepted: September 08, 2009

Hypohydration is a contributing factor to rises in core body temperature and therefore predisposes athletes exercising in hot, humid conditions to the onset of exertional heat illness1–3 and subsequent loss of playing time, altered performance, and decreased overall health.2 To prevent and recognize the occurrence of exertional heat illness, health care professionals should be proactive with populations at risk for exertional heat illness. Prophylactically measuring hydration status and monitoring patients between exercise sessions is required to prevent exertional heat illness, particularly in susceptible individuals.

To accurately assess hydration status, practitioners depend on valid and reliable instruments that are not easily influenced by external interference.4 Although a gold standard for field testing of hydration status using urine has been somewhat elusive,5 urine osmolality is considered the best clinical measure of total solute concentration in urine.4,6 Measuring urine osmolality requires expensive laboratory equipment, trained laboratory technicians, and more time than the alternative field measures.7 Clinicians require accurate, practical, and cost-effective methods of estimating urine concentration in the athletic training clinical setting. Urine specific gravity, urine color, and change in body mass are all common clinical measures of hydration status used in the field1,3,8; each method presents advantages and limitations. In cases in which a valid euhydrated body mass measure is available, researchers suggest body mass change is the most effective method of monitoring hydration status when measured consistently over the course of several days of repeated exercise sessions.3 However, urine indices also are valid methods of measuring preparticipation hydration status and monitoring hydration status throughout a series of exercise sessions.1,3,9,10

Although the National Athletic Trainers’ Association (NATA) and the American College of Sports Medicine suggest the use of a clinical refractometer,1,3 which is supported by other research,4,11–14 urine reagent strips are still widely used among many health care professionals.15–17 Previous research6,9,11–15 has compared urine reagent strips to clinical refractometry; however, these investigations have failed to draw comparisons to the criterion standard of urine osmolality. The purpose of this research was to determine the validity of common clinical measures of urine (clinical refractometer, urine reagent strips, and urine color) compared to urine osmolality.

Methods

Research Design

This study used a nonexperimental, observational research design to compare urine osmolality to a clinical refractometer, 2 brands of urine reagent strips, and urine color. The Heat Illness Index Score (HIIS) Risk Assessment18 was used to identify at-risk athletes on a Division I-A collegiate American football team. The HIIS is a 10-item questionnaire designed to identify risk factors of exertional heat illness during the preparticipation physical examination.18 This screening instrument was examined by 6 expert panelists in a Delphi panel investigation to estimate content validity of the instrument.18 Using this instrument, 5 at-risk athletes were identified, and samples of their urine were obtained before and after practice during 15 days of preseason football practice.

Participants

During the pilot implementation of the HIIS Risk Assessment, athletic trainers identified 5 players at moderate risk for exertional heat illness (mean age, 20.4±0.5 years; mean body weight = 118.7±17.8 kg; mean height = 183.6±5.8 cm). Hydration status was monitored in these at-risk players throughout preseason football practices in a hot, humid environment (mean on-field wet-bulb globe temperature = 28.84°±2.36°C).

Participants were informed of the risks and benefits of participation, and all participants completed an informed consent form, approved by the university’s Institutional Review Board. The goal was to capture 150 samples during the course of the 15 days of preseason football practices, but due to participant noncompliance, a total of 69 samples were collected and analyzed. Only between measurement techniques for each individual urine sample were compared rather than comparisons between participants or samples; therefore the conditions (eg, time of day, level of dehydration, prepractice or postpractice) of data acquisition were not controlled.

Instrumentation

Urine Osmolality. An osmometer (μOsmette; Precision Systems, Natick, Mass) was used with the freezing point depression technique to measure urine osmolality. Osmolality, dependent on the number of particles in the solution but not affected by the weight or charge of the particles,4 is the most effective measurement of total solute concentration and the best method of estimating kidney function.7 An osmometer is used to compare the specimen to the freezing point of water (1.86°C).4 Urine osmolality is far more expensive, requires technical knowledge to perform, and is not a likely instrument to be used in the athletic training clinical or field setting. Because urine osmolality is the criterion standard for estimating urine concentration, it was used as the criterion measure in this investigation. The osmometer was calibrated at the beginning of each data collection session according to the manufacturer’s instructions.

Urine Specific Gravity. Urine specific gravity is the ratio of densities between urine and water2,4,7,9 and has been described as the most practical, cost-effective measurement of hydration status.10 Normal urine specific gravity ranges between 1.002 and 1.030.9 However, the concentration of urine varies because of electrolytes, phosphate, urea, uric acid, proteins, and glucose present.2,11 Three measures of urine specific gravity were used in this study: clinical refractometer (model 300CL Atago USA Inc., Bellvue, Wash), Multistix (Bayer Corporation, Elkhart, Ind) reagent strip (urine reagent strip 1), and Chemstrip 10 (Roche Diagnostics, Indianapolis, Ind) reagent strip (urine reagent strip 2). Measuring urine specific gravity is rapid, noninvasive, inexpensive, and requires only small volumes of urine.7,11 Refractometry is the method of viewing fluid under normal light to detect particles within the fluid.11 The primary investigator LE viewed the fluid and reported urine specific gravity according to gravitational weight of the concentrated fluid.11

Urine reagent strips are an alternative method of estimating urine specific gravity2 and are cost effective, easily accessible, and easy to use.11,14 Researchers immersed the urine reagent strips in a small volume of urine, creating an analytic reaction. The hydrogen ions and sodium ions of the urine react with the ion exchanger (bromothymol) and buffers of the strip. Protons are released in the presence of cations and react with bromothymol blue, changing the color of the strip.2,4,11,14

Urine Color. Urine color was measured using a urine color chart (Human Kinetics, Champaign, Ill). Urine color is an inexpensive clinical method for hydration measurement.7 The urine sample is subjectively compared to the urine color chart and a score is recorded as a measure of hydration status. Normal urine color should be described as light yellow2,7 or a score between 1 and 2 on the urine color chart, whereas significant or severe hypohydration should be described as brownish green or a score between 5 and 6, or ≥6, respectively, on the urine color chart.7

Experimental Procedures

Participants were asked to provide urine samples before and after each practice session (15 days of preseason) in specimen containers and delivered the samples to the mobile field laboratory located outside the locker room. Urine specific gravity was immediately measured using the 3 measures (clinical refractometer and 2 different urine reagent strips). To measure urine specific gravity with the urine reagent strips, one investigator immersed the strip in the specimen and read the results at 45 or 60 seconds by comparing the colored strips to the chart provided on the bottle label. The same investigator measured urine color immediately after receiving the specimen using the urine color chart. Specimens were maintained at room temperature throughout each practice session and then immediately transported to the research laboratory where the primary researcher LE performed the assessment of urine osmolality. These methods of analyzing urine osmolality are consistent with previous research that established urine osmolality as means of assessing hydration status in athletes in the heat, where samples were analyzed within 24 hours of voiding.6

Statistical Analysis

Descriptive statistics were calculated for all urine indices. Urine osmolality, urine specific gravity, and urine color were measured for each individual sample, and correlations between measurement techniques were calculated. Because the scale of measurements for urine osmolality, urine specific gravity, and urine color are different, both Pearson and Spearman’s rho correlations were used to describe the strength of relationships.19 Spearman’s rho correlations were used for calculations including urine reagent strip measures because of the ordinal characteristics of the variable. Significance was set a priori at P ≤ .05.

Results

Measures of urine indices demonstrated participants were significantly hypohydrated (Table 1) according to the standards described by the NATA position statement on Fluid Replacement.1 A strong correlation was found between urine osmolality and the clinical refractometer (Figure), a marked degree of correlation was found between urine osmolality and both reagent strips, and a moderate correlation was found between urine osmolality and urine color (Table 2).

Measures of Hydration Statusa

Table 1: Measures of Hydration Status

Linear Relationship Between Urine Osmolality and Refractometer Measures of Urine Specific Gravity.

Figure. Linear Relationship Between Urine Osmolality and Refractometer Measures of Urine Specific Gravity.

Correlation of Clinical Urine Measures to Urine Osmolality

Table 2: Correlation of Clinical Urine Measures to Urine Osmolality

Discussion

Urine osmolality is considered the standard for measuring the total solute concentration of urine in a research or laboratory setting,4,20 yet urine specific gravity and urine color have been regarded as suitable methods of measuring urine concentration in the clinical or field setting.1 Furthermore, previous investigations only compared urine reagent strips to urine specific gravity, not to the criterion urine osmolality. This investigation revealed strong correlations between refractometry and urine osmolality but only moderately strong and moderate correlations between urine osmolality and urine reagent strips and urine color, respectively. Our findings suggest clinicians should use the clinical refractometer to assess hydration status in conjunction with body mass changes, as a single “best indicator” of hydration status in the field or clinical setting has not yet been established.4,5

Urine Specific Gravity

Refractometry is the preferred method of measuring urine specific gravity in the field or clinic, more so than urine reagent strips.13,14 Yet, the NATA’s Educational Competencies and Proficiencies (4th edition)21 continue to endorse the use of urine reagent strips as a suitable method for measuring hydration status, probably because of the ease and simplicity of urine reagent strips. Research suggests refractometry is a more sensitive indicator of hydration status than blood plasma or hematocrit measurements.22–24 Refractometry is consistently more accurate than urine reagent strips, which often have unpredictable results,4 as confirmed in this investigation. Because a variety of manufacturers produce urine reagent strips, each with product-specific instructions for immersion and scheduled timing for reading results, practitioners may misunderstand or fail to follow the specific directions.14,25 When the urine reagent strip is left in the specimen longer than specified time, the reagents dissolve and become inaccurate.14 Also, urine reagent strips become nonreactive when exposed to ambient moisture, heat, and light, and they also become inaccurate when expired.25

Researchers have compared urine reagent strips and refractometry in a laboratory setting in participants who experienced minimal hypohydration and found that urine reagent strips and refractometry were poorly correlated (r = .573),9 similar to our findings in a clinical setting, where participants tend to be far more hypohydrated. Other researchers have suggested urine reagent strip values ≤1.010 μg correspond better to the refractometer and demonstrate less of a correlation ≥1.010 μg.26 One investigation implied a strong relationship between urine reagent strips and refractometry (r = .906 and r = .911) 15; however, current research has demonstrated correlations well below the clinical standard, r = .800.9,12,25–30 Furthermore, these investigations reveal little about the effective use of urine reagent strips as the comparison provides information about the relationship with refractometry. To identify the validity of urine reagent strips, researchers should compare urine reagent strips to urine osmolality, the standard measure for urine concentration.4 Our investigation revealed moderately strong correlations, which supports that refractometry should be the preferred clinical or field method over urine reagent strips for measuring urine specific gravity.9,12,25–30

Urine Color

According to previous research, urine color does not provide the accuracy or precision of urine specific gravity.7 Our findings suggest urine color is even less effective than urine reagent strips and therefore is not the most effective method of clinically measuring hydration status. However, urine color may be an effective tool in educating athletes about assessing their own hydration status and on further investigation, may be a valid self-assessment of hydration.

Limitations

This investigation focused on clinical or field measures of hydration status during both the preparticipation physical examination and during the course of several preseason football practices. During the preparticipation physical examination, which was a single-measurement session, we were unable to measure body weight changes or total body water losses. Furthermore, as this was a field experiment, we did not use blood measures, such as plasma osmolality, to confirm our results. We believe our results have clear clinical implications for the practitioner, and combined with additional field measures of hydration status such as body mass changes, our findings suggest refractometry is the most valid field measure for urine concentration.

Clinical Implications

Preparticipation hydration assessment is one of many intrinsic characteristics that can effectively identify at-risk athletes and may be helpful in preventing heat-related illness. Educators are responsible for advising future clinicians in selecting an instrument that is practical and inexpensive, and does not require technical operation yet provides valid measurements of urine concentration in the athletic training setting. Our findings suggest the clinical refractometer is a valid clinical measure of urine specific gravity and should be used for pre-participation assessment of hydration status. Furthermore, refractometry measures of urine specific gravity should be used in conjunction with change in body mass and urine color to monitor the hydration status of at-risk athletes exercising in hot, humid environments.

References

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Measures of Hydration Statusa

URINE INDICESMEANSD
Urine osmolality (mOsm/kg)795258
Urine specific gravity
  Clinical refractometer1.0260.010
  Urine reagent strip 11.0220.007
  Urine reagent strip 21.0190.008
  Urine color (shades)62

Correlation of Clinical Urine Measures to Urine Osmolality

MEASURErr2POWER (1-β)
Clinical refractometer.872.7611.000
Urine reagent strip 1.647.419.999
Urine reagent strip 2.626.392.995
Urine color.540.292.909
Authors

Dr Eberman is from the Department of Athletic Training, Indiana State University, Terre Haute, Ind; Ms Minton is from the Department of Physical Education, University of South Carolina, Columbia, SC; and Dr Cleary is from the Department of Kinesiology and Rehabilitation, University of Hawaii at Manoa, Honolulu, Hawaii.

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Lindsey E. Eberman PhD, ATC, LAT, Department of Athletic Training, Indiana State University, 401 North 4th Street, Arena C-10, Terre Haute, IN 47809; e-mail: leberman@indstate.edu

10.3928/19425864-20091020-01

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