URINE TESTS

At present, urinalysis is one of the most common laboratory tests performed. Chemically impregnated strips allow the laboratorian to screen the urine for chemical constituents. Microscopic evaluation of sediment helps to determine the cellular constituents of urine. Routine urinalysis is an excellent screening tool for many different diseases or conditions, such as diabetes, urinary tract infections, metabolic disorders, and liver disorders.

When a urinalysis is performed, the specific gravity of the urine is measured. The specific gravity is important because it is an indication of the quantity of dissolved solids in urine, such as urea and chloride.

The refractometer, or total solids (TS) meter (Fig 8–1), measures specific gravity indirectly by the refractive index of the urine. The refractive index is a ratio of the velocity of light in air to the velocity of light in solution. The angle at which the light passes through the solution is mathematically converted into units of specific gravity. Only one drop of urine is required with a refractometer. The refractometer can be calibrated with deionized water to read 1.000 and 5.00%^(w/v) NaCl to read 1.022.

Correction for Protein

The refractometer must be corrected if large quantities of protein (1.0 g/dL or more) are present in the urine. Protein is a very high molecular weight compound that will increase the density of urine by 0.003 for each 1.0 g/dL of protein. Protein is not normally found in urine because it cannot pass through the glomerular membrane. When the integrity of the glomerulus is compromised because of injury or disease, protein molecules can pass through the glomerulus and will be present in urine. The purpose of performing the specific gravity analysis of the urine is to determine the concentrating ability of the kidney, which primarily is the work of the tubules. The presence of large amounts of protein (1.0 g/dL or more) in the urine will falsely increase the specific gravity and result in erroneous assessment of the tubules.

Example 8–2

A urinalysis was performed on urine that contained 3 g/dL of protein. The specific gravity by refractometer was 1.026. What is the corrected specific gravity for this sample?

Each gram per deciliter of protein falsely elevates the specific gravity by 0.003. Because the urine contains 3 g/dL, the amount to be subtracted is 3×0.003=0.009

Therefore, the specific gravity of the sample that reflects the concentrating ability of the kidneys is 1.017.

Correction for Glucose

The glucose molecule is a large molecular weight compound. When present in large quantities (1 g/dL or greater), the specific gravity of the urine will be increased by 0.004 for every 1 g/dL of glucose. As with protein, increased glucose levels do not reflect the concentrating ability of the tubules. The effect of 1 g/dL of glucose or higher must be subtracted from the specific gravity to obtain an accurate measurement.

Example 8–3

A urine specimen has a glucose concentration of 2 g/dL and a specific gravity of 1.018. What is the corrected specific gravity?

For each 1.0 g/dL of glucose, the specific gravity is elevated by 0.004. The corrected specific gravity would be as follows:

Therefore, the corrected specific gravity is 1.010.

QUANTITATIVE CHEMICAL ANALYSES

The chemistry tests performed during a urinalysis are qualitative in nature but may indicate a problem if they are abnormal. To follow up on an abnormal result, a physician may request a quantitative analysis of a constituent found in urine. For example, a 24-hour sample collected for total protein analysis or for urea may be ordered. The urine sample is analyzed for the particular constituent, but additional mathematical calculations must be performed before the result is recorded. Because the volume and length of collection of the urine sample is variable, the urine result must be standardized to allow comparison of results to occur. Most urine results are recorded as quantity of analyte per unit of time: usually 24 hours or 1 day. Occasionally a urine sample will be collected for a shorter period. In that case, the urine can be reported as “quantity per volume of collection” or as “quantity per unit of time.”

Calculating the Analyte Concentration per Volume of Collection of Urine Analytes

In general, if a nonelectrolyte is measured in urine, it is reported in units of milligrams per day or in the international units of millimoles per day. Urine electrolytes are generally reported in units of milliequivalents per day or millimoles per day. If urine is not collected for a full 24-hour, or 1-day, period, the results may be extrapolated for a 24-hour collection for compounds that are uniformly excreted during a 24-hour period and reported in terms of per-volume quantity collected or quantity per total time collected. To calculate the quantity per volume of collection, a ratio and proportion calculation can be performed.

Example 8–4

A urine specimen collected over 12 hours with a volume of 800.0 mL is analyzed for creatinine. The creatinine result is 90.0 mg/dL. What is the creatinine result in terms of milligrams per volume of collection?

To solve this problem, a ratio and proportion calculation is performed. Remember: units must be the same; therefore, convert deciliter into milliliter terms (1 dL = 100 mL).

Crossmultiplying, the equation yields:

Therefore, there is 720 mg creatinine per 800 mL of urine.

Additional Examples

 Calculate the amount of creatinine in terms of milligrams per volume of collection in a 24-hour urine specimen with a total volume of 1250.0 mL and a urine creatinine of 110 mg/dL.

Substitute into the equation above the values given in this problem to solve:

Solving for X yields a result of 1375 mg creatinine per 1250 mL of urine.

 Calculate the amount of urea in terms of milligrams per volume of collection in a 24-hour urine specimen, with a total volume of 1100 mL and a urine urea concentration of 150 mg/dL.

The concentration is 1650 mg of urea per 1100 mL of urine or 1.65 g of urea per 1.1 L of urine.

Converting the Quantity of Analyte Measured in Milligrams per Deciliter into Grams per Day

The quantity of an analyte measured in milligrams per deciliter can be converted to the concentration of analyte in gram units per day by using the following conversion formula. Note that if it is a 1-day collection, the last fraction within the problem becomes 24/24, or 1, and can be dropped from the equation:

(XanalytemgdL)(mLcollected)(1dL100mL)(gram1000mg)(24hrcollectiontime(hr))=g/day

Example 8–6

A urine urea value of 60 mg/dL is obtained from urine collected for 1 day with a volume of 1500 mL. What is the urine urea value in terms of grams per day?

Substituting into the conversion formula, the following formula is derived:

Therefore, the urine urea concentration is 0.9 g/day or 0.9 g/24 hr of collection.

Additional Examples

 Calculate the urine urea value in terms of grams per day given a urine urea value of 75 mg/dL from a urine sample collected for 1 day with a volume of 2100 mL.

Using the formula shown in Example 8–6 and substituting into it the values from this problem:

The result is 1.6 g/day.

 Calculate the urine protein value in terms of grams per day given a urine protein value of 400 mg/dL from a 1-day urine sample with a volume of 550 mL.

The answer is 2.2 g/day.

One of the most common renal function tests is the clearance test. This test provides information on the glomerular and tubular function of the kidneys. The kidney’s main function is to excrete waste products while reabsorbing water and dissolved chemicals from the ultrafiltrate. By measuring the concentration per unit of time in urine of a chemical that will be removed or “cleared” by the kidney tubules, the physiologic function of the tubules can be determined.

There are many variations on the clearance test. All of them measure three parameters: the plasma (or serum) concentration of the chemical to be cleared, the urine concentration of that same chemical, and the time interval of the clearance procedure. In this manner, the glomerular filtration rate (GFR) can be determined. This is the rate at which chemicals are filtered or “cleared” from the kidney. The clearance tests differ from one another by the chemical to be cleared by the kidney and analyzed. Measurements of chemicals that are foreign to the body and completely cleared are known as extrinsic clearance tests. These chemicals include inulin and p-aminohippurate, among others. The chemical is injected into the patient intravenously. After the chemical is injected, the plasma concentration is measured. All urine is collected during the clearance procedure and the volume measured. After the time interval, which may be 6, 12, or 24 hours, the urine and plasma are analyzed for the chemical concentration. Intrinsic clearance tests measure chemicals that are intrinsic to the body, such as creatinine and urea. The urea clearance test was one of the first clearance tests performed. Because approximately 40% of urea is reabsorbed into the tubules, the urea clearance test does not provide a full clearance assessment. Creatinine is a waste product of muscle metabolism and is produced at a constant rate and proportional to muscle mass. Very little creatinine is secreted by the tubular cells; therefore, the concentration of creatinine in urine is an excellent assessment of the tubular excretion function and the GFR. As in extrinsic clearance tests, the plasma levels of creatinine or urea are determined. Urine is collected for a fixed period ranging from 6, 12, to 24 hours. The 24-hour urine collection is the most commonly performed interval. The urine concentration of urea or creatinine is also determined.

All clearances are calculated by the following formula:

UVP

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