Significance of Urine Analysis in the Diagnosis of Urinary Tract Infections.
J.P. Pagès, DMV, Diplomate ECVIM
St. Orens de Gameville
Urinary tract infections are the second infectious trouble which affects pets just after dermatologic affections. It is considered that 15% dogs and 1 to 3% cats have at least one urinary infection in their life time.
Two types of errors may be made in establishing the diagnosis: false positive animals diagnosed from clinical findings only and from a wrong interpretation of biological findings, and non identified infections due to the lack of systematic urinalysis.
Urinalysis is to be performed in order to analyze the physical, chemical, cytologic and microbiological features of urine. This includes several steps such as collecting urine, examining it, realizing a reagent strip, assessing urine specific gravity by refractometry, performing a nitric acid test to assess proteinuria, realizing urine sediment.
Use of urinalysis for the diagnosis of urinary tract infections
Identification of organisms in urine is the only determining factor in the diagnosis of urinary tract infections but it is the last step of urinalysis. The analysis must be conducted in a way which allows to suspect, confirm, locate and determine predisposing causes. Diagnosis cannot rest on urinalysis only.
Quality of urine samples and their preservation
Physical features of urine
Turbidity most often indicates the presence of solid particles suspended in urine. They may be crystals, cells, organisms, casts, mucin and lipid droplets. Turbidity suggests to perform urine sediment.
An abnormal colour may be the sign of urinary infection: blue (Pseudomonas), red (blood), lactescent white (pus).
Certain odors may indicate urinary infections: ammoniac (bacteria producing urease), putrid smell of proteins deteriorated by bacteria (gross hematuria and blood clots), a smell of hydrogen sulfide which may be persistent in case of urinary tract infection caused by Proteus.
A number of renal or extra-renal infections which may be associated with urinary tract infections manifest themselves by a significant decrease of urine specific gravity <1,015. In Cushing's disease 40 to 50% patients have a urinary tract infection when they are first presented. Pyometras are frequently associated with a urinary tract infection. Excessive urine dilutions may demonstrate upper urinary tract infections (chronic pyelonephritis, chronic interstitial nephritis). However high urine specific gravity (<1,035) may be associated with pyelonephritis or renal infections when one kidney only is affected.
When urine specific gravity is under 1,005, the dilution of bacteria compels to perform a urine culture.
Urine Ph: Alkaline urine is never synonymous with urinary tract infection. In basic urine, a coccus generally is an Streptococcus, a bacillus an E. coli . In acid urine, a coccus is a Staphylococcus, a bacillus a Proteus.
Chemical features of urine
When glycemia is normal, glucosuria demonstrates a deficit in tubular reabsorption: renal disease from infectious origin (pyelonephritis, interstitial nephritis), tubular nephrosis (tubulo-interstitial amyloidosis in 25% cases), tubular necrosis. Glucosuria with high glycemia may be associated with urinary tract infections (diabetes and Cushing's disease). 5 to 10% of patients affected by Cushing's disease are hyperglycaemic. Urinary tract infection is the main cause of ketoacidosis complications in diabetics.
The association of active urine sediment most often demonstrates a post-glomerular proteinuria. When there is paraproteinuria there is bacteriuria without leukocyturia. A certain number of cases of glomerular proteinuria are associated with urinary tract infections. 40% of patients affected by glomerular amyloidosis have urinary tract infections which are often asymptomatic. Urinary tract infections may reveal the renal lesion and be the cause of renal failure. In Cushing's disease proteinuria demonstrates a glomerular lesion. In this disease immunodepression may result in chronic pyelonephritis. 5% of patients with Cushing's disease have phosphate or calcium oxalate lithiasis.
In dogs nitrite test reaction is not a very perceptible finding but is very specific to urinary tract infection.
Urine sediment findings
Urine sediment is to be performed in the presence of turbidity, abnormal colour or odor, proteinuria, glucosuria, peroxidase or esterase positive reactions, nitrites and low urine specific gravity (<1,015)
Organisms (coccus, bacilli, mycelium hypha) are easy enough to detect when their concentration is high enough (104 bacilli and 105 coccus /ml), except with urine with a specific gravity under 1,005 and with non identifiable germ (mycoplasma).
With asymptomatic urinary tract infections, there is bacteriuria without leukocyturia (no inflammation). They may demonstrate immunodepression.
Crystals: Crystals predispose to, but are not the sign of urinary tract infections.
Casts: Leukocytic, hematic casts with bacteria indicates the tubular origin of inflammation or infection.
Abnormal cells: It is sometimes difficult to make the difference between a tumoral cell and a very dysplastic cell. Number of chronic cystitis have pseudo-tumoral macroscopic aspects. Tumors of the urinary tract predispose to urinary tract infections.
Urinalysis is really crucial for identifying urinary tract infections. This examination, when it is performed systematically allows to diagnose asymptomatic urinary tract infections and to discard a number of false positive patients. It also allows to identify the predisposing causes for it allows to suspect renal and extra-renal diseases. It is a simple, quick, cheap examination which must be performed with rigor and skill.