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CLIA Complexity: Moderate




I.       Principle


The refractive index of urine correlates closely with the specific gravity. It is a ratio of the velocity of light in air to the velocity of light in solution, and is directly proportional to the number of dissolved solids in urine.  This ratio varies directly with the number of dissolved particles in urine and, as such, varies similarly with the specific gravity of urine.


II.      Equipment



Disposable pipet


III.     Specimen


Follow policy for random urine collection.  Prior to collecting the patient’s sample, the patient’s identity must be verified by looking at the arm band.  Collect urine in a clean container and test it as soon as possible.  Do not centrifuge or use preservative.  Refrigerate urine if testing is delayed.


IV.     Procedure


1.       Place a drop of urine in the designated spot on top of the refractometer.

2.       Hold the instrument toward a source of light and read the value directly from the specific gravity scale.

3.       Clean the refractometer after each use with  distilled water or weak bleach solution (Quanto), wipe lens with lens paper or soft gauze.


V.       Results


Report result to the nearest thousandth, (e.g., 1.010.)

Results greater than 1.035 should be reported as > 1.035.


VI.     Quality Control


1.       Reagents


§         Sterile water

§         Quantimetrix Normal Urine control - Supplied liquid, ready-to-use, requiring no reconstitution.  It is prepared from human urine fortified to target levels with compounds that produce the desired reaction when tested.  Preservatives including sodium azide have been added to inhibit microbial growth.

§         MAS Level 1 Liquid Urinalysis Control Solution


2.       Storage of Quantimetrix Urine Control


a.       The Liquid controls are stable until expiration date stated on label when stored between 2°and 8°C. Controls can be stored at room temperature for up to one month; record date opened and expiration date on dropper vials on initial use. DO NOT FREEZE.

b.       Discard the control if turbid or any evidence of microbial contamination is present.  Reddish brown uric acid crystals may form in the urine matrix and precipitate due to changes in the temperature.  These crystals will not affect the performance of the control material.


3.       Daily Quality Control


Once a day, a normal and abnormal urine control should be tested on the refractometer.

a.       On initial use, remove the controls from the refrigerator and allow to reach room temperature prior to testing.

          b.       Remove cap and invert bottle.  Apply urine control to refractometer as you would a patient sample.

c.       Read the refractometer according to the procedure above. Record result on the Quality Control Log Sheet.

d.       Wipe off dropper tips and recap control.  The controls can be stored at room temperature.  Record room temperature expiration date in the space provided on the bottle label.


Once a month, the refractometer is checked for accuracy by measuring the specific gravity of water (considered a standard) which should read 1.000 ±0.001.  To verify the accuracy at a high specific gravity, the MAS Level 1 Liquid Urinalysis Control Solution is run.


4.       Expected Range


Compare the lot number given on the quality control log with the lot number on the bottle of urine control material; they should match.  The expected results for the lot are designated on the log sheet for each testing parameter.


If the readings with water consistently depart from zero more than 0.001, the zero setting may need adjustment.  Consult operative manual for instructions.


          5.       Troubleshooting

          a.       Clean refractometer and repeat control.

b.       If control still out-of-range, check control for expiration date, inspect instrument, review technique.

c.       Repeat control.

d.       If control still out-of-range, open a new bottle of control solution and repeat test.

e.       If control comes in range, discard controls that gave out-of-range results.  If controls are still out-of-range, go to step f.

f.        Do not use instrument, call or page the Point-of-Care Testing Coordinator at ext. 4431 or pgr. 4431.






VII.    Proficiency Testing


As an additional assurance of quality, two proficiency testing samples will be distributed three times annually.  These samples will be from the Clinical Microscopy Survey (CM) from CAP and will be distributed by the Pathology Department.  Only one select nursing unit will perform the survey per PT event.  Results of the PT will be reported on the Urinalysis Quality Management Report.


VIII.   Reference Range


Specific gravity = 1.003-1.030

The refractometer gives values for specific gravity comparable to those obtained by other methods with the advantage of using only one drop of urine.  Results greater than 1.035 should be reported as > 1.035.  Critical Values are up to the assessment of the physician.


IX.      Limitations


IVP dyes interfere with specific gravity resulting in abnormally high readings.  If the reading goes beyond the scale, dilute the urine 1:2 and remeasure.  The presence of protein and glucose in urine also increases the specific gravity.


X.       Maintenance


1.       Daily


a.       Clean the eyepiece, prism glass and coverplate with lens paper or soft gauze.  Any smears on any of these parts interfere with readings.  Do not use any abrasive material or scratchy paper to clean these parts.


2.       Quarterly


Remove the rubber holder and wash it out to make sure urine does not collect inside and corrode the refractometer.


3.       Other


a.       The meter is compensated to give correct readings directly on aqueous solutions at all temperatures ranging from 15°C to 37°C.  Never expose instrument to temperatures above 65°C.

b.       The instrument is damaged by immersion of the eyepiece and focusing ring in water.

c.       An air bubble to accommodate thermal expansion of liquid is kept in a bubble trap at the end of the refractometer cavity.  If it escapes and appears in the visible part of the prism, hold the instrument vertically, eyepiece down, and shake lightly.





XI.      Competency Checks


Competency checks are performed at initial orientation, after 6 months of employment, and  yearly thereafter by the POC Medical Technologist or a validated nurse.


XII.    References:


1.       Freeman JA, Beeler MF: Laboratory Medicine: Clinical Microscopy. Lea and Febiger, Philadelphia, 1974:181-189.

2.       Ames Company: Modern Urinalysis. Ames Company, 1973:19-22.

3.       Davidson I, Henry JB (eds.): Clinical Diagnosis by Laboratory Methods, 15th ed. WB Saunders, Philadelphia, 1974:45-47.

4.       Lippman RW: Urine and Urinary Sediment. Charles C. Thomas, Springfield, IL. 1957:101-103.

5.       The dropper Plus, Package Insert, Quantimetrix Corporation, Hawthorne, CA. October, 1993.