Urinalysis: Collection, Specific Gravity, Dipstick and Sediment

Introduction

A urinalysis is one of the most common tests a veterinary technician runs in the clinic. It is part of the minimum database, along with the complete blood count and chemistry panel, and it tells the veterinarian about the urinary tract and about other body systems, such as the liver and red cell destruction.¹¹²

A complete urinalysis has four parts: physical (visual) assessment, urine specific gravity (USG), the dipstick (chemical) test and the sediment exam under the microscope.³ This guide covers how to collect and store the sample, how each part is done, which dipstick pads you can trust in animals, and how to recognize crystals, casts and cells. Most errors in urinalysis come from handling, not from the patient, so technique matters.

Key facts

How the VTNE tests this

• Perform the test: Domain 4 Task C lists urinalysis among the laboratory tests a technician performs. K24 covers sample collection, handling, storage and shipping; K25 covers laboratory methods; K27 covers normal and abnormal results.¹⁵
• Choose the collection method: which method to use for a culture, and how each method changes cells and bacteria in the sediment.
• Identify: a crystal, cast or cell from a description or image of its shape.
• Troubleshoot: why an old or cold sample gives wrong results, or why a dipstick pad should be ignored.
• Calculate: a USG from a diluted sample that was off the refractometer scale.

Core concepts

How the collection method changes the result

Always record how the urine was collected, because the method changes what you see.²

• Free catch (voided): easy and safe. A mid-stream catch reduces contamination, but bacteria, squamous cells and debris from the distal urethra, genital tract and skin can still appear.²¹² Give owners a clean container with a tight lid; containers from home may hold detergent, food or drug residue.¹² Urine from the floor or litter box is the poorest sample and may contain fecal material.²
• Catheterization: a sterile catheter is passed after aseptic preparation. It can slough extra transitional epithelial cells and cause mild bleeding, and it can carry bacteria into the bladder.²¹²
• Cystocentesis: a needle is passed through the body wall into the bladder. It is the cleanest method and the preferred one for culture.²¹² It can cause microscopic hematuria: the urine still looks yellow, yet more than 100 RBC per high power field may be seen.²

Bacteria in a cystocentesis sample are significant at any number, because bladder urine is normally sterile. Small numbers of bacteria in a voided or catheter sample may be contamination.¹⁹

Time and temperature

Urine changes as it sits. With delay, pH rises, bacteria multiply, cells and casts break down, and bilirubin and ketones degrade.² Merck advises evaluating urine within 30 minutes; if that is not possible, refrigerate it.¹ Refrigeration does not alter pH or specific gravity, but it can make crystals precipitate.¹ Calcium oxalate and struvite crystals can both develop during storage, even if they were absent in the fresh sample.² Always warm refrigerated urine to room temperature before testing; USG and the dipstick need room-temperature urine.¹³

The time of day also matters. Morning (fasting) samples tend to have a higher USG, a lower pH and more formed elements than afternoon samples.²

Urine specific gravity

USG compares the density of urine with that of pure water, so it has no units.⁴¹¹ It shows whether the kidneys can concentrate or dilute urine. The veterinarian always reads it alongside hydration status and blood urea nitrogen and creatinine.¹⁴ Use a refractometer made for veterinary samples. Many have a separate scale for cats; use the dog scale for other species.¹

Large amounts of glucose or protein in the urine falsely raise the USG reading.⁴¹¹ Radiographic contrast agents such as iohexol can raise it too, for 24 to 48 hours after a study.⁴ This is one reason urine should ideally be collected before treatment, including IV fluids, is started.³

Source: eClinPath, Concentrating ability (Cornell University).⁴ Cutoffs vary between references, so use your laboratory's values.

The dipstick (reagent strip)

A dipstick is a plastic strip with pads that change color when an analyte is present. Results are semiquantitative (negative, trace, 1+ and so on).¹³ The strips are designed for human urine. In animals you report pH, protein, glucose, ketones, bilirubin and blood (heme).¹⁵ A 2023 study in dogs found poor agreement between visual and analyzer readings for the leukocyte pad and for specific gravity.¹³

Sources: Merck Veterinary Manual, Urinalysis; eClinPath, Chemical constituents and Overview; Smyroglou and others, 2023.¹³⁵¹³

Protein must always be read against USG. A trace of protein in concentrated urine matters less than the same trace in dilute urine.¹ In urine with a USG below 1.015, even trace to 1+ protein is excessive.⁵ When the veterinarian needs a number, the urine protein to creatinine ratio (UP:UC) is measured on a chemistry analyzer.⁵

Glucose normally stays out of urine because the kidney reabsorbs it until blood glucose passes the renal threshold: above 180 mg/dL in most species and above 240 mg/dL in cats.¹ If the glucose pad is positive, a blood glucose should be checked.¹

A positive blood pad can mean intact red cells, hemoglobin or myoglobin. Look at the sediment: red cells mean hematuria. With no red cells, look at the plasma. Pink to red plasma points to hemoglobin from red cells broken down in the blood vessels; clear plasma points to myoglobin from muscle.¹

Sediment: cells, casts and organisms

Report red and white cells as the number per high power field (HPF, 40× objective). Up to about 5 RBC/HPF and fewer than 5 WBC/HPF are commonly accepted as normal, though this depends on the collection method.⁸ White cells are a little larger than red cells and have grainy cytoplasm.¹ Transitional epithelial cells line the bladder and proximal urethra; they look like large white cells. Squamous epithelial cells are large flat cells seen in voided samples.¹⁸ Fat droplets are common in dog and cat urine and can be mistaken for red cells; they vary in size and float in a different plane of focus.¹ Sperm may be normal in urine from intact male dogs.¹

Casts are cylinders of mucoprotein that form in the renal tubules and may trap cells. Count them per low power field (LPF, 10× objective).¹⁷ Hyaline casts are clear, with parallel sides and rounded ends; they are hard to see without lowering the light. A few hyaline or granular casts can be normal. Cellular, coarsely granular and waxy casts, even in low numbers, indicate kidney damage.¹⁷ Casts dissolve with time, especially in dilute or alkaline urine, so a delayed sample can hide them.⁷

Small amorphous crystals and debris can look like bacteria. A Gram stain helps, and aerobic culture is best to confirm a bacterial infection.¹⁶

Crystals

Crystals form when urine is supersaturated with a substance. Which type forms depends on urine pH, concentration, temperature, and time since collection.¹⁶ Report them as few, moderate or many. Judge large crystals such as struvite at low power and small crystals such as calcium oxalate dihydrate at high power.⁶

Sources: eClinPath, Crystals; Merck Veterinary Manual, Urinalysis.¹⁶ Merck gives the pH range for struvite (alkaline) and cystine (acidic).

Two points matter most. First, crystalluria is not the same as urolithiasis: crystals can appear without stones and stones can form without crystals.¹ Second, crystals can form in the tube after collection, so a fresh sample examined at room temperature gives the truest picture.¹⁶ In dogs, struvite crystals without a urease-producing bacterial infection are not linked to struvite stones; cats can form struvite stones without infection.¹

The ethylene glycol (antifreeze) link is the urgent one. Its toxic metabolite, oxalate, binds calcium and forms calcium oxalate crystals in the renal tubules.¹⁴ Picket fence calcium oxalate monohydrate crystals are frequently associated with ethylene glycol poisoning in dogs and cats, but they are not seen in every case.⁶ If you see them, tell the veterinarian at once; treatment is time critical (see the emergency and toxicology guides in Domain 8).¹⁴

Procedure or technique

Assisting with cystocentesis (blind technique)

1. Restrain the patient in lateral recumbency; some dogs can be sampled standing.¹²
2. The person sampling palpates and isolates the bladder in the caudal abdomen to make sure it holds enough urine.¹²
3. The needle is inserted at about a 45 degree angle without suction. At the right depth the plunger is drawn back slowly.¹²
4. Pressure on the plunger is released before the needle is withdrawn.¹²
5. For culture, change the needle before transferring urine into a tube, and refrigerate the sample until it is shipped.¹²

Ultrasound-guided cystocentesis follows the same steps while the bladder is viewed on screen.¹² Urine leak and bleeding after cystocentesis are rare.¹²

Running the urinalysis

1. Label and record the patient, date, time and collection method.²
2. Physical exam: mix the sample and record color and turbidity (clear, slightly cloudy, cloudy, opaque or flocculent). Normal urine is yellow to amber and clear.¹¹⁰ Color is subjective and is not a substitute for USG.⁴
3. Dipstick: use uncentrifuged, room-temperature urine. Dip the strip briefly or drip urine on each pad, blot the edge on gauze, and hold the strip flat so urine does not run between pads. Read each pad at the time the maker states. If the urine is very bloody or discolored, use the supernatant after spinning.¹³ Keep the bottle tightly capped and discard expired strips.¹
4. Centrifuge 3 to 5 mL of urine in a conical tube at low speed, about 1,000 to 1,500 rpm for 3 to 5 minutes.¹ Try to use the same volume every time, because the semiquantitative sediment counts depend on it.³
5. USG: put 1 to 5 drops of supernatant on the calibrated refractometer prism, close the cover and read the line where light meets dark.³¹¹ Clean the prism with distilled water and a soft cloth after each use.¹¹
6. Prepare the sediment: pour off the supernatant, leaving about 0.5 mL. Resuspend the pellet by tapping the tube, place a drop on a slide and add a coverslip.¹
7. Examine unstained, with the condenser lowered or the iris diaphragm closed for contrast. Scan the whole coverslip at 10× for casts, large crystals and parasite eggs, then check structures and at least several fields at 40× for cells, small crystals and bacteria.¹³ Stains such as new methylene blue help identify cells but can add precipitate and dilute the sample.¹

Flowchart

Species differences

• Dog: urine is usually acidic.¹ A small amount of bilirubin, and a few bilirubin crystals, can be normal in concentrated urine.¹⁶ Dalmatians and English Bulldogs can have ammonium biurate crystals without liver disease.⁶ Male dogs are the easiest patients to catheterize.²
• Cat: urine is usually acidic and more concentrated than dog urine, with a usual USG of 1.035 to 1.060.¹⁴ Read USG on the cat scale of a veterinary refractometer.¹ Bilirubin in cat urine is always abnormal.⁶ The renal threshold for glucose is higher than in dogs, above 240 mg/dL.¹
• Horse: urine is usually alkaline and is often cloudy from mucus and calcium carbonate crystals, which are normal.¹⁶¹⁰ Equine urine can turn brown after standing.¹
• Ruminants: urine is usually alkaline. Calcium carbonate crystals are normal in goats, and calcium oxalate crystals are common in healthy cattle.¹⁶ Ketonuria is a sign of ketosis in ruminants.¹
• Rabbits and guinea pigs: calcium carbonate crystals are normal.⁶

Calculation

Safety, legal and client education

• Wear gloves when handling urine; Papajeski lists gloves for protection against zoonotic disease.¹¹
• Do not clean the refractometer prism with abrasive cleaners or cloths; scratches ruin readings.¹¹¹²
• Owners collecting at home: give a clean container with a tight lid, ask for a mid-stream sample, and ask them to bring it in quickly or refrigerate it.¹¹²
• Do not use plastic red-top tubes for storing urine; they can introduce crystal-like artifacts. Glass plain tubes or sterile containers are suitable.²
• When shipping, tell the outside laboratory if any preservative such as formalin was added, because it affects staining.¹
• Antifreeze is a household poison. If a client mentions possible exposure, alert the veterinarian immediately.¹⁴

Common mistakes and exam traps

Quick review

1. A urinalysis has four parts: physical exam, USG, dipstick and sediment.
2. Cystocentesis is the cleanest method and the first choice for culture.
3. Bacteria in a cystocentesis sample are significant at any number.
4. Examine within 30 minutes or refrigerate, then warm to room temperature.
5. Storage raises pH, grows bacteria, breaks down casts and cells, and forms crystals.
6. Calibrate the refractometer to 1.000 with distilled water; read the cat scale for cats.
7. Isosthenuria is 1.008 to 1.012; hyposthenuria is below 1.008.
8. Ignore the dipstick SG, leukocyte, nitrite and urobilinogen pads in animals.
9. Read dipstick protein against USG; trace protein in dilute urine is significant.
10. Spin 3 to 5 mL at low speed, examine unstained with the condenser lowered, 10× then 40×.
11. Count casts per LPF and cells per HPF; cellular and waxy casts mean kidney damage.
12. Struvite is a coffin lid, CaOx dihydrate an envelope, CaOx monohydrate a picket fence.