THE INVESTIGATION OF URINARY TRACT INFECTIONS (UTI)

Definition 

The term urinary tract infection encompasses a broad spectrum of infectious processes involving the urinary tract.

Initial laboratory investigations

Pretreatment midstream urine (MSU) for MCS

Dipstick screening - because the human eye cannot 
distinguish colour changes reliably, these chemical urinalysis tests must be read by an appropriate instrument. 

Pyuria can be detected by microscopic examination or dipstick for leucocyte esterase.
N.B. Dipstick leucocyte esterase has a sensitivity of 75 - 90%.  Dipstick nitrite is used as a surrogate marker for bacteriuria; not all uropathogens, e.g.  enterococci, 
S saprophyticus reduce nitrates to nitrites and therefore give false-negative results. A positive test for nitrite, leucocyte esterase or protein is not diagnostic of significant bacteriuria, and requires urine culture for confirmation.
Gram stain of unspun urine is time consuming, has low sensitivity, and is therefore not performed in most clinical laboratories unless specifically requested.

Urological work-up , in selected patients only e.g. children, most adult men, women with relapsing  infection, patients with recurrent pyelonephritis. Includes: Cystourethrogram Cystoscopy Renal ultrasonography IVP CT scan

CONDITION

SPECIFIC LABORATORY TESTS

Acute uncomplicated cystitis in women

Pretreatment midstream urine (MSU) for MCS

E. coli (90%)
S. saprophyticus (8%
P. mirabilis 
K. pneumoniae

Those most at risk are sexually active women. Most UTIs in this population are uncomplicated and are rarely associated with functional or anatomic abnormalities. Aggressive diagnostic work-ups in young women presenting with an uncomplicated episode of cystitis is unwarranted.

Recurrent cystitis in women 

Pretreatment midstream urine (MSU) for MCS - the causative organism should be identified to differentiate between relapse (infection with the same organism) and reinfection (infection with different organisms). Multiple infections with the same organism (relapse) are, by definition, complicated UTIs (see below) and require further diagnostic tests. Most recurrent UTIs in women are uncomplicated infection caused by different organisms, and are generally not associated with underlying abnormalities and do not require further work-up of the genitourinary tract.

Complicated UTI 

Anatomic or functional factors predispose the patient to recurrent or persistent infection, or treatment failure.

Enlargement of the prostate Urinary tract obstruction Indwelling catheters Organism resistant to multiple antibiotics Vesicoureteral reflux
Calculi 

Accurate urine MCS and susceptibility studies PLUS Follow-up urine cultures 

Persistence of organisms necessitates urological work-up of the patient.

Acute pyelonephritis 

Should always be considered in patients who have severe rigors, hypotension, and/or shock.

Pretreatment midstream urine (MSU) for MCS including microscopy for casts. 

E. coli 
P. mirabilis 
K. pneumoniae
S. saprophyticus is a rare cause

Blood cultures - positive in up to 20%

Renal ultrasonography or CT scan if symptoms persist after 3 days of therapy to exclude urinary obstruction or abscess.

Asymptomatic bacteriuria 

Pretreatment midstream urine (MSU) for MCS

Pregnant women

In pregnancy, a screening urine culture should be obtained towards the end of the first trimester of pregnancy. If screening urine culture is normal, a seconding urine screening should be offered in the third trimester. If screening positive, a monthly follow-up urine culture is recommended to detect asymptomatic relapsing bacteriuria. 

Renal transplants
Genitourinary procedures

UTI in men

Suspect underlying prostatic disease in men with symptoms and signs of UTI.

Pretreatment midstream urine (MSU) for MCS Urology evaluation is recommended

Prostate disease
Urinary outlet obstruction

Catheter-associated UTI 

Urine MCS

UTI in children 

Pretreatment midstream urine (MSU) for MCS

Anatomical abnormality esp. vesicoureteral reflux

Either midstream, suprapubic or sample collected by placing a bag over the perineum (infants).  Urological work-up including a voiding cystourethrogram, ultrasonography.

REFERENCES

1. Saint S, Scholes D et al. The effectiveness of a clinical practice guideline for the management of presumed uncomplicated urinary tract infection in women. American Journal of Medicine 1999; 106: 636 - 641
2. Mandell, Douglas and Bennett. Principles and Practice of Infectious Diseases 2000 773 -805
3. Holland DJ, Bliss K et al. Comparison of Chemical Dipsticks read visually or by photometry in the routine screening of urine specimens in the clinical microbiology laboratory. Pathology 1995; 27: 91 - 96