Taking a dip into urinalysis

Transcription

1 Vet Times The website for the veterinary profession Taking a dip into urinalysis Author : Christine Jameison Categories : RVNs Date : July 1, 2009 Christine Jameison RVN, probes into the extensive subject of urinalysis, focusing on getting the best results from dipstick tests URINALYSIS can be one of the most valuable tools in diagnosing and managing certain conditions, and involves gross examination, dipstick tests, specific gravity measurements (using a refractometer) and sediment examination under the microscope. The subject of urinalysis is long and complex; therefore, for the purpose of this article I will focus on the the use of dipstick tests. To start with, the urine must of course be collected. It is generally advised to use an early morning sample as this reflects all the urine produced overnight, and can eliminate hourly changes. Do remember, though, that cells will degenerate in urine stored in the bladder for a long period of time and, therefore, it is not advised to use this sample for microscopy/sediment evaluation. There are various methods of collection a midstream free catch, manual compression, catheterisation and cystocentesis. All of these are acceptable for a chemical urinalysis; however, it is essential to try to test the freshest sample possible, typically within one to two hours of collection. If sampling is not possible within this time scale, a form of preservation is required. Refrigeration is always recommended and will allow you to test the urine up to six to eight hours after collection. Boric acid may also be used to preserve samples, at a concentration of 0.8 per cent, and, apart from the ph, this should not interfere with a dipstick analysis. If the sample has been refrigerated, it must be allowed to return to room temperature before any analysis is performed. Testing the sample when it is cold can cause crystal formation, which alters 1 / 6

2 the appearance. The dipsticks are enzymatic and, therefore, using them when cold can give falsely low results. Dipstick analysis Most people tend to stick the whole strip into the urine sample; however, this is not advisable. Each pad contains specific chemicals that react with the urine to give the colour change. Dipping the stick in the urine increases the chance of colours running, which can give false readings. The preferred method is to apply a single drop of urine on to each reagent pad using a plastic pipette, and allowing this to stand for a few seconds before turning the strip upside down (over a sink) to remove excess urine. Always follow the specific timings and instructions for the type/make of dipstick strip, and remember to check which way the strip has to be read, that is, from left to right, or bottom to top, etc. The majority of reagent pads are enzyme based and are therefore sensitive to time and temperature. There are many dipstick strips on the market and, although most have a similar substance measure, they can have different pad reaction methods. Many of these strips are designed for use in humans and, therefore, false positive readings may occur (see later for specific examples). Grossly turbid samples, especially those that are haematuric, will alter the colour change of the pads, giving false readings. It is advisable to either allow the sample to sediment out or centrifuge the sample and test the supernatant. The normal urine dipstick tests for the following substances: glucose, ketones, bilirubin, urobilinogen, blood, protein, ph, specific gravity, nitrite and leukocytes. Glucose A small amount of glucose is normally present in urine, but the levels are usually insufficient to be detected and, therefore, the normal reading should be negative. It is useful to know what container the urine was collected in especially for samples brought from a client s home as some, such as jam jars, may lead to false positive readings. Positive readings could be stress induced (especially in cats) or indicate diabetes mellitus or proximal renal tubular dysfunction, but be aware that some medication can lead to a positive glucose reaction. Any positive result should be confirmed with blood analysis. If the blood glucose is negative and the urine is still positive this could indicate kidney or renal losses. Certain container types, cats with cystitis and some medications can lead to false positive results. 2 / 6

3 False negative results may be seen with large quantities of ascorbic acid excreted in the urine. Ketones Ketones are not normally seen in urine. If they are detected but glucose is not, excessive fat metabolism is indicated and, as long as the patient is well, should not be significant. Positive readings will be associated with uncontrolled diabetes mellitus, hyperthyroidism, fever and starvation, which leads to carbohydrate restriction. Bilirubin Normal dogs (especially males) have small amounts of bilirubin present in their urine, especially if the specific gravity is greater or equal to Positive results may indicate biliary obstruction, liver disease, hepatic haemolysis and cholestasis. False positives are possible, especially with haematuric samples. Bilirubin detection is very uncommon in cats; if discovered during urinalysis it should not be ignored. Urobilinogen This is a metabolite of bilirubin and is produced by the action of anaerobic bacteria. This is a more useful test when screening human patients, hence, it being present on the dipstick, but has no relevance in companion animals. Blood The reagent strip will detect intact red blood cells, haemoglobin and myoglobin, although most cannot differentiate between the various components. Indications for the presence of these are listed in Table 1. Myoglobinemia is rare in dogs and cats and a suspected positive result should be accompanied with clear serum and evidence of muscle trauma or disease before this condition is even suspected. Haemoglobinemia is sometimes detected on the dipstick, but if the urine sample is not red in colour, it is unlikely to be a true result. Blood should not be present in urine, although certain collection methods, such as cystocentesis, may result in blood being recorded. If haematuria is suspected, this should always be confirmed 3 / 6

4 using microscopy. False positives are possible if the sample has been taken from a recently disinfected surface, or if the sample is very alkaline, this can contribute to red blood cell lysis and therefore give false positive readings for haemoglobinemia. As a general rule, any positive result needs to be investigated further with either microscopy or additional tests. Protein Protein is not normally present in urine. Trace amounts may be seen if the specific gravity is greater than and also if the urine is very alkaline (ph>8) false positives may occur. The more commonly used dipsticks may only be sensitive to albumin, so cannot be used as a true measure of protein levels. The most accurate way to determine protein in urine is to perform a protein: creatinine ratio test. False positives are seen if chlorhexidine has been used either to wash out containers or as a scrub prior to cystocentesis. Very alkaline urine can also produce false positives as the dipstick is designed for human use. See Table 2 for examples of the causes of proteinuria. ph The ph records the acidity or alkalinity of the urine. Normal ph in the dog and cat is between 5.5 and seven, with an average of 6.7. Acidic urine is under seven and alkaline urine is over seven (with ph seven being neutral). Human urine has a ph of eight to nine, which is alkaline and will, therefore, produce false positive readings of protein if patients have alkaline urine. Urine that contains glucose can cause acidification, as can certain diets and diseases. Examples of disorders associated with abnormal ph values are listed below. Acidic urine The causes of acidic urine include the following: respiratory / metabolic acidosis; 4 / 6

5 diabetic acidosis; primary renal failure; severe vomiting; severe diahhreoa; starvation; and pyrexia. Alkaline urine urinary tract infection; respiratory / metabolic alkalosis; vomiting; renal tubular disease; and detergents / disinfectants. A ph meter or litmus paper is more accurate in determining the specific ph of urine samples, especially if the measurement is critical for patient evaluation. Specific gravity Specific gravity measures the concentrating abilities of the kidney tubules. Unfortunately the highest value the reagent strip can detect is to and, therefore, is unsatisfactory when trying to adequately detect renal concentration. Dipstick specific gravity pads are not reliable and a refractometer should be used wherever possible. Remember to always calibrate the refractometer regularly. The typical range for specific gravity on a normally hydrated patient is: Dog: Cat: / 6

6 Powered by TCPDF ( As a rule of thumb, a specific gravity greater than suggests dehydration. Nitrites Nitrates originate from an animal s diet and are a normal finding in urine; however, nitrite, which is the reduced form of nitrate, is not found in urine. Nitrite can also be produced by some bacteria. Positive results can suggest significant bacteriuria, and microscopy should be performed to confirm the presence of bacteria. The urine sample should also be sent to an external laboratory in a boric acid container for culture and sensitivity testing. Leukocytes Not all dipsticks have leukocyte test pads on them. The pad primarily detects neutrophils and, although this is useful in human samples, it should not be solely relied on to identify pyuria or infections. Microscopy must always be performed to confirm or deny those results. False positives are very common in feline samples and examination for white blood cells under the microscope is essential. Positive results usually suggest active inflammation of the urinary or genital tract. Summary Urinalysis is an extensive subject and involves not only the dipstick, but visual inspection of a sample, refractometer readings and microscopy. The dipstick definitely has its place within urinalysis, but the collection method, time of collection, the container used and the general health of the patient must always be kept in mind, before determining the relevance of the results. References Osborne C A and Stevens J B (1999). Urinalysis: A clinical guide to compassionate patient care. Bayer Animal Health, p / 6