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A guide to ... the interpretation of urine testing

Deborah Rigby RGN OND MSc
Quality and Performance Manager
Wiltshire Primary Care Trust

Urine testing is a routine part of patient care and is a simple and quick way of monitoring disease and identifying new illness

The testing of urine to determine health is not a new concept and goes as far back as the Egyptians, who produced writings about polyuria. The smell of urine can be an indication of disease, with infected urine often smelling 'fishy'. Acetone excreted by diabetics with ketoacidosis gives urine a characteristic smell. Colour, taste and smell can all be an indication of a patient's condition, and drugs and diet can cause the urine to change colour.1

In the mid-1960s, scientific developments led to the near-patient testing method of reagent testing, initially in the form of tablets, followed by chemically impregnated reagent sticks. Recent innovations have explored the detection of bladder and prostate cancer using voided urine.2

Urine is a natural waste product of the body and is typically clear, pale or deep yellow in colour, due to a pigment called urochrome, derived from the body's destruction of haemoglobin. The more concentrated urine is, the deeper yellow it becomes and changes in colour may reflect diet, medication or may be due to blood or bile in the urine (see Box 1).

[[Box 1. Urine]]

Often, the detail of urinalysis can be neglected and the significant information can be overlooked or misinterpreted. The reasons for inadequate urinalysis include:
•    Improper collection.
•    Timing of examination.
•    Near-patient testing method.
•    Interpretation of microbiology findings.

Improper collection
The problem of urine testing can often be the sample and its method of collection, storage and time interval before testing. The container must be clean, dry and free from contaminants (eg, jam in jam jars), and the specimen must be collected correctly. Hands should be washed before and after collection. Cleansing with water does not reduce contamination and antiseptic leads to a false negative result.

In women, the specimen should be taken mid-stream. Holding the labia apart reduces contamination but may not be practical. In toddlers, potties washed in hot water (60°C with washing up liquid) are suitable for collection. In infants, a urine collection pad in a nappy may be used. All specimens should be refrigerated to prevent bacterial overgrowth or stored in a specimen pot containing boric acid as a preservative. For a quick reference guide produced by the Health Protection Agency see Resources.

Timing of examination
A first voided specimen (early morning) can be helpful to ensure accuracy and some studies suggest the urine should have been stored in the bladder for at least four hours to guarantee an accurate result. The sample must not be left lying around after collection because bacteria in the urine, whether causing an infection or merely an accidental contamination, may grow, causing changes in pH, degrading other constituents of the sample and confusing the result.

The bottle should be filled to the marked line if it contains boric acid. This is because a low volume of urine can create a concentration of boric acid high enough to be bactericidal. Refrigerate urine at 4°C while awaiting processing. Urine that has been stored at 4°C for 48 hours is suitable for culture but not for microscopy, because most cells will have disintegrated.3,4 Urine preserved with boric acid may be stored for up to 96 hours.5

Local policy should be followed with regard to correct handling of the sample and guidance from infection control policies. If the specimen is disposed of and not sent to the laboratory, correct procedure is to use an infected waste bin and slice facilities (not down the sink).
  
Near-patient testing
Near-patient testing is a term used to describe a clinical test that takes place near the patient, using chemically impregnated reagent strips for urine testing, which are now more sophisticated. The reagent sticks are considered reliable if stored out of direct sunlight, kept at a constant temperature, in date and used according to manufacturers' instructions.
Infection of the urinary tract usually produces pus cells that release an esterase (leukocyte esterase), which react with the leukocyte reagent pad. The presence of leukocytes is, therefore, an indication of UTI. Nitrite is not normally present in urine and nitrites are produced by the bacterial breakdown of dietary nitrate, which is a waste product of protein metabolism.

Considerable controversy exists around the accuracy of reagent testing. The negative predictive values (NPV) define the proportion of individuals with a negative diptest who do not have an infection. Flannagan et al and Hiscoke et al suggested that urine samples that are visually clear and negative to dipstick testing for blood, protein, nitrites and leukocytes could be discarded as uninfected based on negative predictive values of 92% and 98%.6,7 Recent studies by Sundvall and Gunnarsson showed that when dipstick urinalysis for nitrite and leukocyte esterase are both negative it is unlikely that the urine culture will show growth of potentially pathogenic bacteria and, in a patient with an uncomplicated illness, further testing is unnecessary.8

Interpretation of microbiology findings
Bacterial infection
Urinary tract infections (UTIs) are among the most common infections seen in hospitals and general practice, but the clinical diagnosis of UTI is often considered imprecise.9 Bacteriuria is defined as the presence of bacturia in the urine, when the specimen has been contaminated with peri-urethra flora during collection. Infection is confirmed by counting the number of bacteria.

Significant bacteriuria is defined as the presence of more than 10/5 organisms per ml of urine. It is reported that distinguishing between 10/3 and 10/5 can be difficult, making it important to collect the sample carefully and transport rapidly to the laboratory.
 
White blood cells
More than five to eight white blood cells per high power field is generally considered abnormal. Pyuria refers to the presence of abnormal numbers of leukocytes (pus cells) and may appear with infection. White cells from the vagina and cervical infections and external meatus in men and women may contaminate the urine. Consider chlamydia trachomaris infection if the patient is 16-24 years old, or in the existence of other vaginal infections, other non-cultured organisms or renal pathology.

Khasriya scrutinised the sensitivity and specificity of dipstick urinalysis and microscopic pyuria (10 or more white blood cells per mcl) to identify infection.10 The study concluded concern over established testing of microscopic pyuria for diagnosing urinary tract infection in some patients.

Red blood cells
The presence of red blood cells in the urine is abnormal, as heamaturia associated with cystitis or urethritis is generally clear after treatment. Persistent heamaturia should be investigated further.

Epithelial cells
Squamous epithelial cells in urine usually indicate contamination of the specimen from the distal urethra in men and introitus 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 exhibit abnormal histology they may be indicative of a malignant process affecting the lining of the bladder (urothelium).

[[Tab 1. Urine]]

Conclusion
Urine testing is a simple procedure and a very valuable tool that yields important information about disease, from a UTI to life-threatening conditions such as diabetes, cancer of the bladder or renal disease. To get more out of our humble urine testing we need to be aware of the correct process for collecting urine samples and reduce the risk of contamination.

Interpreting results requires knowledge and skill and, like most tools, the more you know about it the more valuable it can become. To give our patients the best possible service we must be able to use these results properly. We must also remember the importance of looking at the patient holistically, taking into account the results of further tests, where indicated, as well as the patient's own views about their wellbeing and lifestyle.

References
1.    Ford H. Feeling off colour: colour of urine and faeces indicate         disease. Nurs Times 1992;89(5);64-8.
2.     Sullivan PS, Chan JB, Levin MR, Rao J. Urine cytology and adjunct markers for detection and surveillance of bladder cancer. Am J Transl Res 2010;2(4):412-40.
3.     Sanderson PJ. Laboratory methods. In: Brumfitt W, Hamilton-Miller J, Bailey RR, eds. Urinary tract infections. London: Chapman and Hall Medical.
4.     Horton JA 3rd, Kirshblum SC, Linsenmeyer TA, Johnston M, Rustagi A. Does refrigeration alter culture results in hospitalised patients with neurogenic bladders? J Spinal Cord Med 1998;21(4):342-7.
5.     Health Protection Agency (HPA). Investigation of urine. National Standard Method BSOP 41. London: HPA; 2008. Available from: www.hpa-standardmethods.org.uk/documents/bsop/pdf/bsop41.pdf
6.     Flannagan P, Davies E, Rooney PG, Stout RW. Evaluation of four screening tests for bacteriuria in elderly people. Lancet 1989;1(8647):1117-9.
7.     Hiscoke C, Yoxall H, Greig D, Lightfoot NF. Validation of a method for the rapid diagnosis of urinary tract infection suitable for general use in general practice. Br J Gen Practice 1990;40(339):403-5.
8.     Sundvall P, Gunnarsson R. Evaluation of dipstick analysis among elderly residents to detect bacteriuria: a cross-sectional study in 32 nursing homes. BMC Geriatr 2009;9:32.
9.     Aliyue SH, Ludlum H, Abubakar I, Bentley N. What is the role of urine dipstick testing in the management of UTI? Br J Gen Pract 2002;52(478):414-15.
10.     Khasriya R, Khan S, Lunawat R et al. The inadequacy of urinary dipstick and microscopy as surrogate markers of urinary tract infection in urological outpatients with lower urinary tract symptoms without acute frequency and dysuria. J Urol 2010;183(5):1843-7.

Resources
Health Protection Agency (HPA)
Diagnosis of UTI: Quick Reference Guide for Primary Care
www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1194947404720

Further reading
Lister S, Dougherty L. The Royal Marsden Hospital Manual of Clinical Nursing Procedures, 8th edn. London: Wiley-Blackwell; 2010.