Urine Specific Gravity
Urinalysis has been shown to be the most valid and reliable method for determining moderate changes in fluid balance. Here is a method of analyzing urine specific gravity using a refractometer.
- purpose: monitoring hydration levels to prevent dehydration is important for optimizing performance. Urine specific gravity is a scientific measure of hydration by measuring the density (concentration) of a urine sample.
- equipment required: a refractometer (a simple hand-held version is illustrated here), urine specimen containers for urine collection, distilled water, cleaning cloth / disposable tissues, fridge or ice cooler for urine storage, gloves.
- Collecting the urine. The first part of the urine stream is discarded, then a small sample of urine is collected into a container. The sample can be measured immediately or stored for later measurement.
- Calibrating the refractometer. Calibrate the refractometer by placing distilled water on the glass as the sample, and adjusting the scale to read 1.000. This should be done before you begin testing, and after every ten samples or so to ensure that the calibration remains accurate.
- Measurement. Open up the flap at the end of the refractometer. Clean with distilled water and dry with a soft non-abrasive cloth. Place a drop of urine on the glass plate and close the flap. Hold the refractometer up towards an area of natural light, look though the eye piece and read the specific gravity level off the scale - the point where the contrast line (difference between light and dark areas) crosses the scale.
- results: The measurement may be done immediately after collection, or the specimen can be stored in refrigeration for later analysis. The specific gravity results will range from 1.000 (which is equivalent to water) up to 1.035 (very dehydrated). There are several levels that are used in the literature to indicate dehydration, such as a value of 1.15 or greater.
- comments: The sample is usually collected first thing in the morning. It may also be of interest to collect samples prior to or post exercise, though there may be a time delay for the effect of dehydration to show in the specific gravity measure.
- certain medicines, vitamins or the presence of glucose may cause the urine specific gravity to change and give incorrect readings of dehydration. If any of these situations occur then the test is unreliable.
- The refractometer should be calibrated before you begin testing, and after every ten samples or so to ensure that the calibration remains accurate.
- advantages: The hand held refractometer is very easy to operate.
- disadvantages: This test requires the collection of urine (which is sometimes difficult) and the purchase of a specific apparatus for measurement. For a more simple test of hydration you can use urine color.
- Comments: There is a minimal difference in the accuracy of the related measures of urine specific gravity, urine osmolarity, and urine color (Armstrong et al. 1998).
- Reference: Armstrong, L.E., Soto, J.A., Hacker, F.T., Casa, D.J., Kavouras, S.A., Maresh, C.M. (1998). "Urinary indices during dehydration, exercise, and rehydration." Int. J. Sport Nutr. 8: 345-355.
- hydration for sport: a collection of articles
- about measuring dehydration
- other methods: urine color measurement, body weight changes, sweat analysis, sweat rate