ABSTRACT: The aim of this article is to introduce nurses to some of the common artefacts found in urine samples. These artefacts can have a detrimental effect on the results and the way they are interpreted. This article should, hopefully, provide information on how to spot these artefacts and what can cause them.

Urine analysis is the most common diagnostic tool used in practice and is most often performed by veterinary nurses. It is quick, cost effective and non- invasive, and can flag up abnormalities such as renal impairment and diabetes before running a biochemical test, and can help veterinary surgeons understand more about a medical case.

We can all think of images of marigold- gloved owners being dragged through the undergrowth on the end of an overextended extendable lead trying to catch a few drops of urine in a ‘well washed’ jam jar or old pill pot. Unfortunately, something as simple as a drug residue in the pill pot can seriously affect the urine pH and alter the colour change on dipstick pads.

In order to achieve the maximum information from a urine sample, nurses should be aware of common artefact changes introduced by sampling techniques and sample storage that could possibly lead to a misdiagnosis. All abnormalities should be interpreted in conjunction with each other and with the patient’s history.

Method of sampling

It is important for the veterinary nurse or technician to be aware of the artefacts that can be introduced through the different sampling methods (Figure 1); for example:

Figure 1: Turbid urine from a free catch' sample

• non-midstream free catch samples may include a lot of cells, bacteria and miscellaneous debris from the patients skin and genitalia 

• blood and fat (lipids) can be introduced when taking cystocentesis samples

•  catheterising patients can cause trauma and introduce blood to the urine, leading to a false diagnosis of haematuria.

These artefacts can significantly affect how the results are interpreted, so when sending urine samples to an external laboratory it is very helpful if the method of collection is written on the submission form. It is also very helpful to provide clients with a sterile collection container if they are to collect the sample at home.

Sample storage

How the sample is stored is as important as the collection method – the older the sample, the less accurate the results become. Obviously, the quicker the sample is tested the better; but if the sample has to be stored overnight, it is useful to know that its specific gravity (SG) and most biochemical parameters are stable for 6-12 hours.

It is important that the sample be allowed to return to room temperature before testing takes place, because very cold urine will slow the response of the reagent pads on the dipstick, leading to false negatives.

Ideally, you should perform the sediment exam on a ‘fresh as possible sample’ as prolonged refrigeration or storage can reduce the viability of some micro¬organisms for culture and also allow some crystals to form. If a culture is to be performed, then using a boric acid collection pot is advisable. Table 1 lists some common artefacts caused by sample ‘ageing’.

Dipsticks

A quick mention should be given to the dipsticks used in veterinary practice.

Most dipsticks are designed for human use, and the reagent pads for SG, nitrite, urobilinogen and leukocytes can be very misleading by giving falsely high or low results. These should be ignored, if possible.

Confirmation of the SG can be done using a refractometer and a sediment examination should be performed to check for leukocytes (Figures 2 & 3).

Figure 2: Cocci bacteria, white blood cells and an epithelial cell

Figure 3: White blood cells and a sheet of epithelial cells

Urine that is highly icteric or contains a high level of blood cells will physically obscure most of the reagent pads on the dipstick, leading to false positives for protein and false negatives for glucose (Figure 4).

Figure 4: Haematuria

Over the years, I have seen people use dipsticks in a variety of fashions. In my opinion, the best way is to drop urine on to each reagent pad using a pipette and then gently tap the urine off sideways to prevent cross-pad contamination.

Conclusion

I hope, from reading this article, you will come to realise that how the sample is handled and stored is almost as important as the results that you obtain. Poorly handled samples will give you misleading results and possibly an incorrect diagnosis.

Sediment examinations should be performed on fresh samples, and if the sample has been refrigerated, then it must be allowed to return to room temperature before testing. If the sample is being sent to an external laboratory, then please indicate on the accompanying form how it has been stored and how it was collected, so the results can be interpreted accordingly. 

Author

Matthew Garland CertNatSci(Open) VN MBVNA

After qualifying as a VN in 2004, Matthew worked in small animal practice before moving to Torrance Diamond Diagnostic services in 2006. Now, as laboratory manager of TDDS-Ringwood, he has developed a strong interest in haematology and biochemistry.

To cite this article use either

DOI: 10.1111/j.2045-0648.2012.00227.x or Veterinary Nursing Journal Vol 27 pp 378-379

Further reading

BSAVA Manual of Clinical Pathology [2nd edition] Eds Elizabeth Villers &Laura Blackwood.

Veterinary Nursing Journal • VOL 27 • October 2012 •