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Understanding urine testing.

VOL: 101, ISSUE: 12, PAGE NO: 60

Debbie Rigby, MSc, RGN, OND, FEATC, is continence service manager, Bristol South and West Primary Care Trust

Karen Gray, RGN, FPCert, Asthma Dip, is specialist practitioner (practice nursing) and practice teacher (SCPT), Battersea, London

Debbie Rigby, MSc, RGN, OND, FEATC, is continence service manager, Bristol South and West Primary Care Trust

 

Urine is typically clear, a pale to deep yellow colour and slightly acidic (pH6). This colour is due to the presence of urochrome, a pigment derived from the destruction of haemoglobin. The more concentrated urine is, the deeper yellow it becomes.

 

 

Changes in colour may reflect dietary intake - for example, consumption of beetroot may result in urine becoming red; this may also be due to the intake of certain medications, or abnormal constituents such as blood or bile in the urine.

 

 

The smell of urine can indicate disease. For example, infected urine often smells fishy, and acetone excreted by patients with diabetic ketoacidosis gives urine a sweet smell (Box 1) (Dougherty and Lister, 2004).

 

 

Specimen collection
The methods for urine specimen collection are listed in Box 2. Urine for routine near-patient urinalysis (the clinical test takes place near the patient) should be collected in a clean, dry container. The patient and/or the nurse should wash their hands before and after the specimen is collected.

 

 

The procedure for collection of a mid-stream specimen of urine for laboratory investigation is outlined in Box 3. It is vital the procedure is explained to the patient and that consent has been given.

 

 

Local policy should be followed for the handling of samples, and unused specimens should be disposed of using infected waste bins and sluice facilities; they should not be thrown down the sink.

 

 

Timing for specimen collection
It is suggested that first-voided specimens (early morning ones) that have been stored in the bladder for at least four hours provide the most accurate results.

 

 

The sample should be tested as soon as possible after collection. Bacteria in the urine, whether they are causing an infection or merely causing accidental contamination, may increase in number over time, cause changes in pH or degrade other constituents of the sample, all of which can affect the results of the urine test.

 

 

Near-patient testing
Near-patient testing is a term used to describe a clinical test that takes place near the patient. Examples include blood-sugar monitoring, blood-pressure measurement and temperature recording. Methods for this type of urine testing were developed in the mid-1960s. Initially, reagent tests involved tablets that were added to the patient’s urine.

 

 

This method was followed by the development of chemically impregnated reagent sticks. These were initially used as screening tools for protein or glucose in the urine, but are now more sophisticated and are used for testing for a range of urine constituents (Box 4).

 

 

Reagent sticks should be:

 

 

- Stored out of direct sunlight;

 

 

- Kept at a constant temperature;

 

 

- Not used after their expiry date;

 

 

- Used according to manufacturer’s instructions.

 

 

There is disagreement over the accuracy of reagent testing and studies have suggested the urine test is only 85 per cent reliable and that its use in screening patients is questionable (Yuen and Ng, 2001; Preston et al, 1999).

 

 

Reagent sticks for detecting urinary tract infections - Infection of the urinary tract usually produces pus cells that release an esterase (leukocyte esterase). This can be detected by using a urine reagent stick.

 

 

Likewise, nitrites can be detected using a urine reagent stick. Nitrites are not normally present in urine; they are produced by the bacterial breakdown of dietary nitrate, which is a waste product of protein metabolism.

 

 

Urine reagent sticks have been reported as being useful tools for excluding the diagnosis of a urinary tract infection based on their high negative predictive value (NPV). The NPV defines the number of individuals with a negative test who do not have an infection.

 

 

Flannagan et al (1989) and Hiscoke et al (1990) suggested that urine samples that are visually clear and negative to reagent stick testing for blood, protein, nitrites and leukocytes could be discarded as uninfected based on negative predictive values of 92 per cent and 98 per cent respectively.

 

 

The role of urine reagent sticks as tools to reduce laboratory workload has also been reviewed. Bowler et al (1998) introduced local guidelines that recommended the use of urine reagent sticks as a screening tool for urinary tract infection. As a result they reported a 53 per cent fall in urine samples submitted to a laboratory for microscopy.

 

 

Woodward and Griffiths (1993) noted that the absence of leukocytes and nitrites in a fresh urine sample confirms its sterility, while the presence of one of the markers suggests a possible urinary tract infection. However, conversion of a nitrate to a nitrite by bacteria requires a four-hour incubation period in the bladder and it is therefore not uncommon to have a nitrite negative urine test result that later shows a positive urine culture result (Bayer Diagnostics, 1997).

 

 

False negative nitrite tests may occur in patients taking vitamin C and a false negative leukocyte test can be caused by glucosuria, and drugs such as nitrofurantoin and rifampicin.

 

 

Fenwick et al (2000) questioned the cost-effectiveness of the urine reagent stick test and found empirical treatment with or without laboratory culture to be more cost-effective than reagent stick alone.

 

 

There is a debate about when the urine specimen should be sent for culture, where antibiotic therapy is being considered for the patient (see p69).

 

 

Interpretation of the microbiology results
When a urine specimen is sent to the laboratory for microscopy, culture and sensitivity, the report will identify the presence of bacteria, and white, red and epithelial cells.

 

 

Bacterial infection - Bacteriuria is defined as the presence of bacteria in the urine; however, because the specimen can be contaminated with periurethra flora during collection, infection is confirmed by counting the number of bacteria. Significant bacteriuria is defined as the presence of more than 105 organisms per ml of urine.

 

 

White blood cells - Pyuria refers to the presence of abnormal numbers of white blood cells and these may be present with an infection. White cells from the vagina and cervical infections and external meatus in men and women may contaminate the urine.

 

 

Red blood cells - The presence of even a few red blood cells in the urine is abnormal. Haematuria (blood in urine) associated with cystitis or urethritis will usually resolve after treatment. Persistent haematuria should be investigated, as it may indicate disease such as malignancy.

 

 

Epithelial cells - Squamous epithelial cells in urine usually indicate contamination of the specimen from the distal urethra in men and the opening of the vagina in women. It is not uncommon to find transitional epithelial cells in the normal urine sediment; however, if they are present in large numbers or clumps and have an abnormal histology they may indicate malignancy affecting the urothelium (the lining of the bladder).

 

 

Implications for clinical practice
Urine testing is a simple procedure and a valuable tool, which yields important information about diseases ranging from a urinary tract infection to diabetes, cancer of the bladder or renal disease.

 

 

Effective use of urine testing depends on using the correct procedure to collect the urine sample and the measures adopted to reduce the risk of contamination. Nurses need appropriate knowledge and skills to interpret results.

 

 

To give patients the best possible service, nurses must be able to use the results appropriately. It is important to remember that urine testing should not be viewed in isolation and that results should be reviewed as part of an holistic assessment.

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