Sediment examination in 7 easy steps
Fresh urine samples should be placed into a plain tube and examined on the same day to avoid deterioration and bacterial overgrowth. It can always be refrigerated for a few hours, however, make sure the sample comes back to room temperature before examination. Remember that boric acid collecting pots are only used for samples that will be cultured, as the acid prevents bacterial overgrowth.
- Use a standard volume – this is to allow an accurate comparison between samples, so anything between 5 and 10 mls will be sufficient.
- Spin the sample – take 5 mls of urine and spin at 1500 rpm for 2 minutes to form a sediment.
- Remove the supernatant – this is usually pipetted off, leaving approximately 0.5 ml of a sediment pellet.
- Resuspend the sediment – this is done by holding the tube at the top and flicking the bottom a couple of times.
- Add stains – these are added into the tube, using the same amount each time.
- Add to the examining slide – either use a standard grid slide, which allow an exact volume of urine to be examined under and within a grid for counting, or on a simple slide with a cover slip.
- Microscopy – the sample is then examined at different magnifications. x100 magnification (x10 lens) for the quantification of casts and x400 (x40 lens) for the identification and quantification of cells and crystals.
What can you see under the microscope?
The reasons why so many of us fail to take the time to look at a urine sample under the microscope are varied, but one of them is that we’re not be familiar or confident of what we see. Below is a brief summary of some of the more common features.
A. Cellular content of urine
Cellular constituents that may be present in urine include leukocytes, erythrocytes, epithelial cells and sperm. Epithelial cells are often difficult to see detail in using a sediment stain. A more reliable method is to allow the sediment to dry and then add a standard cytological stain, such as the Diff Quick stain.
- Squamous epithelial cells – these are frequently seen as contaminants in voided urine samples and can also contaminate samples collected by catheterisation. Urine collected by cystocentesis should not contain any squamous epithelial cells. Large numbers may represent abnormal genitourinary conditions, specifically squamous metaplasia of the prostate in the dog. They are large in size, folded and have an angular appearance.
- Transitional cells – these cells naturally slough into the urine in quite low numbers, so only a few would normally be seen in the urine from healthy animals. Look for medium sized cells which are large and round. Large transitional cell counts should make you suspicions of inflammation or neoplasia.
- White blood cells – you would normally expect to see less than 5 WBC per high power field (x400). Larger numbers may suggest an inflammatory process. Pyuria often is caused by a urinary tract infection, so check for bacteria.
- Red blood cells - 5 RBC per high power field or less generally are considered normal. Higher numbers may indicate haemorrhage, inflammation, necrosis, trauma or neoplasia somewhere along the urinary tract.
These are formed from secreted of protein in the tubules and/or cells. When urine transit is delayed through the tubules, the number and nature of casts can change, which allows a more formed cylindrical cast to develop, with cells or cell breakdown products to be impressed into it. These casts can tell us what is going on higher in the tubules. The different casts include;
- Hyaline casts – this mucoprotein forms in healthy animals as well as in animals with a fever, renal disease or on diuretics.
- Granular casts – these are degenerating cellular casts or aggregated proteins. They can form with glomerular disease, tubular disease, pyelonephritis and viral infections.
- Waxy casts – these are the end stages of a granular cast and may be present in advanced renal failure or other conditions associated with dilated tubules and reduced flow.
- Fatty casts – these are lipid containing renal tubular cells and form with nephrotic syndrome.
- Red Blood Cells - red cell casts correspond with renal haematuria rather than haemorrhage from cystitis. They are also present with glomerulonephritis and acute tubular trauma.
- White blood cells – these will be found with pyelonephritis, glomerulonephritis and tubulointerstitial nephritis.
These are a frequent finding in urine samples, and can form for a variety of reasons, such as low or high pH, high concentration, the temperature of the sample and the presence of bacteria.
Common crystals include;
- Struvite – these may be an incidental finding in healthy carnivores.
- Calcium carbonates – you would normally find these in lagomorphs.
- Calcium oxalate monohydrate crystals – these are associated with ethylene glycol poisoning.
- Ammonium biurate crystals – these are associated with poor liver function and portosystemic shunts.
Always be aware that there may be unexpected cells and particles in the sediment which can interfere with your interpretation of the sample. Some of the more frequently found contaminants include;
- Lubricating gel – this can appear under a microscope as a crystalline material.
- Glove powder – this can leave starch granules, which appear as crystals.
- From catheterised animals – these samples may contain squamous epithelial cells from the external skin, genitalia or distal urethra. Free flow samples may in some situations also be contaminated with these cells.
- From samples aspirated off the floor – these can easily be contaminated with parasite eggs, organic debris, fungi and bacteria.