Accurately Diagnosing Bacterial Urinary Infection in Cats

Accurately Diagnosing Bacterial Urinary Infection in Cats

Story at-a-glance -

  • Feline lower urinary tract disease (FLUTD) is the most common health problem of cats, and there are a number of causes, including urinary tract infections.
  • Bacterial urinary infections are rare in otherwise healthy cats, but they present their own set of challenges when it comes to an accurate diagnosis.
  • Urine sediment tests using a wet-unstained method for examination produced significantly inaccurate results when compared to urine culture results. However, tests using the dry-stained method produced results in alignment with culture results.
  • Vets who suspect a bacterial urinary infection as the cause of FLUTD should request dry-stained (air-dried modified Wright-stained) urine sediment tests to more accurately determine the presence of bacteria and the need for a urine culture.

By Dr. Becker

One of the most common disorders affecting kitties is feline lower urinary tract disease (FLUTD).

Also known as feline urologic syndrome (FUS), FLUTD is a group of disorders, any of which can affect your cat's bladder or urethra, including cystitis (inflammation of the bladder), urinary tract infections, urinary stones, urethral plugs, cancer and other disorders.

If your cat begins urinating outside the litter box, the first thing you should assume is you have a medical problem on your hands rather than a behavioral problem.

Only after your vet has checked thoroughly for all forms of FLUTD can you safely assume your kitty's change in elimination habits is behavioral in nature.

Because there are so many different conditions that can contribute to FLUTD, it's extremely important to get an accurate diagnosis of the underlying cause so appropriate treatment can be given.

Bacterial Urinary Infections

A very common cause of lower urinary tract disease in cats is cystitis, which usually resolves on its own within a few days.

However, symptoms of a bacterial infection are very similar to those of cystitis.

And while bacterial urinary tract infections are rare in cats that do not also have diabetes or kidney disease, it's important to know as soon as possible whether a kitty is suffering from cystitis or an infection.

Cystitis can often be effectively treated with natural remedies, including homeopathics (Cantharis is a common remedy), cranberry extract, and TCM nettles tincture. It's important to understand, however, that cystitis is an inflammatory condition, not an infection, and remedies that treat inflammation do not treat infection.

In order to accurately diagnose and treat FLUTD, a urinalysis is required.

A urinalysis can actually involve three separate processes: chemical testing, sediment testing and culturing. Unfortunately, often results of culturing don't agree with sediment test results.

This is a frustrating situation to encounter in the case of suspected bacterial infection, primarily because it can delay appropriate treatment.

It can also delay diagnosis overall. Well over half of all cases of FLUTD are idiopathic in nature, meaning they have no known cause. In order to arrive at that conclusion, every other known cause must be ruled out, including organ disease and metabolic problems, as well as bacterial, fungal and parasitic infections.

Fortunately, there is a way to get a more accurate diagnosis in the case of a bacterial infection, which I'll discuss shortly.

First let's take a look at the three different methods used to analyze a urine sample.

Chemical Testing

Urine samples can be checked for several different substances using chemical reagent test strips. These substances include bilirubin (an indicator of liver disease), glucose (a marker for diabetes) and blood, which is often present when there is infection or inflammation in the body.

Urine test strips can measure all of the following substances:

Glucose pH
Bilirubin Protein
Ketone (Acetoacetic Acid) Urobilinogen
Specific Gravity Nitrites
Blood Leukocytes

Included in urine chemical testing is examination of what is called specific gravity, which is a measure of the concentration of the urine. A very concentrated sample means the specific gravity is too high, which is an indicator of dehydration.

A diluted urine sample means specific gravity is too low, which can point to several different disorders, among them hyperthyroidism, diabetes, and kidney disease.

Specific gravity can also be measured with a tool called a refractometer, which yields the most accurate results.

Urine Sediment Testing

To examine urine sediment, a sample of urine is centrifuged, meaning it is placed into a machine that separates liquids from solids.

The sediment in the urine collects in a layer at the bottom of the test tube. A special dye is then applied to the sediment.

Next the sediment is viewed under a microscope. The examiner is able to check red and white blood cell counts and whether bacteria or mineral crystals are present. Abnormal or unusual cells can also be visualized.


A culture is typically the next step when a urine sediment test shows the presence of organisms. The culture is used to find further evidence of and identify organisms like bacteria, fungi or parasites (bacteria is by far the most common finding) that may be causing a urinary tract infection.

When a urine sample is sent for culture it has to be completely sterile, which means it must be obtained directly from the kitty's bladder.

The procedure to accomplish this is called cystocentesis. A thin needle is inserted through the abdominal wall into the bladder and a small amount of urine is drawn into a syringe. This is a quick procedure which cats tolerate remarkably well.

The sample is sent to a laboratory for culturing of bacteria or other organisms present in the urine. If no organisms grow, the test is considered negative. If organisms grow in sufficient numbers to indicate an infection, the culture is positive.

The type of organism is identified. If bacteria are found, the lab can then test them against several antibiotics (known as a sensitivity test) to determine which drug will be most effective in killing off those particular bugs.

Study Reveals the Method of Urine Sediment Examination is Crucial

In the case of a suspected FLUTD with a bacterial cause, the problem arises when the urine sediment test and urine culture show significantly (sometimes wildly) different results.

In other words, what was seen in the sediment test can't be validated during the culturing process, which leaves everyone scratching their heads as to how to proceed.

In a recent study conducted at Michigan State University, 472 feline urine samples were collected for the purpose of comparing methods of sediment testing vs. culturing results. The two methods of sediment testing were wet-mounted unstained and dry-stained.

Of the 472 samples, 29 had a positive culture for bacteriuria, which is the presence of bacteria in sterile urine (collected by cystocentesis). Since culturing is considered definitive for the presence of bacteria, the 29 positive samples were the 'real deal.'

In the 29 positive samples, 33 different strains of bacteria were isolated. The most common were E. coli (present in almost half the samples), Staphylococcus intermedius, and Enterococcus spp.

Using the wet-unstained sediment test method, 214 of the 472 samples showed the presence of bacteria. With the dry-stained method, bacteria was present in only 30 of the 472 samples – which is obviously much more in line with the 29 positive samples found by culturing.

Per Clinician's Brief:

In the 29 positive culture samples, the dry-stained method had greater morphologic concordance and lower misclassification rates than the wet-unstained method. The dry-stained method using a modified-Wright stain had improved sensitivity, specificity, and overall test efficacy versus the wet-mount unstained method.

What This All Means for You and Your Cat

If your favorite feline develops urinary tract problems and your vet recommends a urine culture to check for the presence of bacteria, you now know there is an additional test that can help validate culture results.

First ask your vet whether a urine sediment test was done, whether it was examined using the more accurate dry-stained method (also called 'air-dried modified Wright-stained'), and if so, what the results revealed.

If the dry-stained method was used to test your cat's urine sediment, and bacteria was noted, there's a very good chance of a true bacterial urinary infection. I would certainly recommend going ahead with the culture at that point so the lab can sensitivity test the bacteria to find the most appropriate medication to use to clear the infection. .

If, on the other hand, you discover the wet-unstained method was used and produced a positive result for bacteria, you might want to discuss your concerns about the accuracy of the test with your vet.

In the MSU study, when compared to the 'real deal' urine culture findings, the wet-unstained method returned the following results:

  • 53.2% true-negative
  • 4.6% true-positive
  • 1.5% false-negative
  • 40.7% false-positive

That's a lot of false positives – almost 41 percent -- for the presence of bacteria. Over 40 percent of the time, this testing method detected bacteria where there was none.

My guess is many veterinarians start cats on antibiotic therapy based on urine sediment test results while waiting for culture results. Given the number of false positives from the wet-unstained method, I can also guess a lot of cats without an infection are taking unnecessary antibiotics.

Now take a look at the dry-stained test method results:

  • 92.6% true-negative
  • 5.1% true-positive
  • 1% false-negative
  • 1.3% false-positive

As you can see, using the dry-stained method, there's much less chance your cat's urine will test positive for bacteria if there's no bacteria present.

If you do find out the less accurate wet-unstained method was used, you have the option to ask your vet to arrange for another urine sediment test at a lab that uses the dry-stained method. Or you can go ahead and ask for a urine culture and sensitivity test, which will give you a definitive answer on the presence of bacteria, and also determine which antibiotic will be most effective against any existing bacteria.

A Word about Treatment and Followup

Although I love to use d-mannose in patients with urinary tract conditions, I've experienced many situations in which it does not resolve infections.

The goal in treating a bacterial urinary infection is to kill off all the bacteria. This typically requires antibiotic therapy. As much as I try to avoid giving animals unnecessary antibiotics, it's important to do so when an infection is present.

At the other extreme of cautious, conservative use of antibiotics to clear up confirmed infections are the vets who grab a box of Clavamox for every case of FLUTD, no matter the underlying cause. This approach is as irresponsible as withholding antibiotics when they are truly necessary. It fosters the escalating problem of 'superbugs' that grow resistant to antibiotics. It can also potentially leave a serious urinary tract issue, like crystals, unresolved and untreated.

One of the most important things you MUST do if your kitty has a bacterial urinary infection is follow up after treatment with a second urinalysis/dry-stained urine sediment test. You want to confirm the infection is completely resolved.

This should be done regardless of what treatment is undertaken to eliminate the infection. Don't assume because the symptoms are gone the infection is. Delaying complete resolution of the infection can result in chronic FLUTD. Untreated, unresolved urinary tract infections can lead to life-threatening kidney disease.

+ Sources and References