Ultrasound-Guided Cystocentesis: How and Why to Do One
There are several ways to collect a urine sample from your patient, including free catching, using a urinary catheter, and doing a cystocentesis.
A cystocentesis—which involves obtaining a urine sample via a needle placed directly into the bladder—may be the preferred method in many cases.
This is especially true when you need a sterile urine sample, such as for culture and sensitivity testing for UTIs. And in general, a cystocentesis will provide more accurate results than a free catch urine sample, since it reduces contamination from cells or pathogens of the skin.
It’s possible to perform a cystocentesis by palpating the bladder or using anatomical landmarks alone. However, an ultrasound can make the process easier by removing the guesswork and allowing you to visualize everything.
Here’s how it works…
How to perform an ultrasound-guided cystocentesis for dogs and cats
Prepare for the procedure.
Use an appropriate size syringe (usually 3ml to 12ml) based on the size of your patient.
Choose a needle size and length based on your patient size. A 22g needle works well for many patients.
Use a towel, trough, or other cushioning to make your patient more comfortable during the procedure. This may help minimize their movement.
Use appropriate patient restraint.
For some dogs and cats, this could mean having a trained team member hold them. For very anxious or fractious patients, this might mean chemical restraint.
Dorsal recumbency is generally the best way to position your patient.
Be sure to clean the area of skin where you’ll be performing the cystocentesis. If needed, consider clipping the fur, too.
Estimate the bladder location.
You’ll use your ultrasound to see the bladder, but anatomical landmarks will give you a good starting point for where to place the probe. Try one of these methods…
Locate the caudal four mammary nipples and mentally draw an ‘X’ between them. The bladder will often be near the cross point of the ‘X’.
Place a small amount of alcohol on the caudal abdomen so it pools. Often, the bladder is located directly under where the alcohol naturally pools on the midline.
Visualize the bladder.
Place your probe onto the caudal abdomen. You should be able to see the urinary bladder on the screen—it’s a fluid-filled structure that has the appearance of a ‘black balloon’ with gray or white walls.
Check for any abnormalities.
You could focus on just doing the cystocentesis. However, while you’re visualizing the bladder, it may help to do a cursory exam (or even a more in-depth evaluation, if indicated) to look for any problems contributing to the patient’s urinary symptoms.
For example, note if you see any thickenings, masses, calculi, or other issues in the urinary bladder.
Obtain the urine sample.
Place the needle directly through the abdominal wall, just in front of the ultrasound probe.
Aim the needle slightly caudally.
You should see the needle on your ultrasound screen. Use this visual to guide you, to be sure you get a good, clean sample rather than aspirating part of the bladder wall.
Prepare your sample for analysis.
Right after the cysto, replace the needle on the syringe with a sterile one.
Then, divide and prepare the sample in a timely manner (the fresher the sample, the better for testing) for any tests you need to perform, whether that’s in-house or at an outside lab.
In many veterinary hospitals, cystocentesis is safely performed several times per day, and complications are very rare. However, as with all medical procedures, some risks exist.
These may include…
Hematuria (usually mild and transient).
Much rarer complications such as bladder injury, leakage of urine into the abdomen, puncture of internal organs or blood vessels, or vagal reactions (retching, panting, hypersalivation, collapse).
Seeding of tumors.
If you suspect your patient has a bladder tumor such as transitional cell carcinoma, it may be best to avoid a cystocentesis. Otherwise, the needle could ‘seed’ the tumor as you withdraw your sample from the bladder, allowing cancer to spread into the abdominal cavity.
An ultrasound may help you to notice abnormalities such as bladder tumors during the procedure, in which case your recommendation to the client may change.
Fortunately for most pets, a cystocentesis is a quick outpatient procedure that provides a lot of valuable information. And many dogs and cats don’t show any side effects at all.
For patients with urinary symptoms, a cystocentesis is often the way to go for the most accurate diagnostic testing results and to get to the bottom of the issue faster for healthy pets and happy clients.
Written by: Dr. Tammy Powell, DVM