Urinary Tract Infection (UTI) : Etiopathogenesis and Lab Diagnosis

2 Anatomic Categories:

1. Upper UTI:

  1. Acute pyelitis (renal pelvis)
  2. Acute pyelonephritis (renal parenchyma)

2. Lower UTI:

  1. Cystitis (urinary bladder)
  2. Urethritis (urethra)

Predisposing Factors:

  1. Obstruction of urinary flow: Benign Prostate Hypertrophy, Calculus, Tumor, etc.
  2. Surgery: on the kidney or urinary tract
  3. Catheters: inserted through the urethra into the bladder
  4. Diabetes Mellitus and Immunocompromised states
  5. Vesicoureteral reflux (VUR): Reflux of urine from bladder up into ureters and renal pelvis
  6. Neurogenic bladder or Bladder diverticulum
  7. Female gender: Incidence in female is greater in male due to
    • Proximity of urethra to anus: Colonization by colonic organisms
    • Short length of urethra: About 4 cm
    • Sexual intercourse: Introduction of bacteria into the bladder
    • Use of diaphragm: Method of contraception
    • Bladder or uterine prolapsed: Postmenopausal
  8. Pregnancy: Upper UTI is common in pregnancy due to
    • Dilation of ureters and renal pelvis
    • Stasis in right ureter
    • Atony in ureteric musculature
    • Incompetence of vesicourethral valves leading to VUR
  9. Congenital abnormalities in Infants

 

Etiology:

Common causative agents:

  1. Escherichia coli (Commonest etiology)
  2. Klebsiella
  3. Proteus mirabilis and other species
  4. Staphylococcus saprophyticus
  5. Enterococci (Group D Streptococci)
  6. Pseudomonas aeruginosa

Infrequent causative agents:

  1. Enterobacter, Citrobacter, Staphylococcus aureus
  2. Candida albicans
  3. Adenovirus type 2
Bacterial Virulence factors:
  1. Increased ability to adhere to urethroepithelial cells: Fimbriae
  2. Increased resistance to serum cidal activity
  3. Hemolysin
  4. Urease
  5. Bacterial motility
  6. Endotoxin production

Mode of Infection:

  1. Ascending infection
  2. Hematogenous route

Clinical Manifestations:

  1. Urinary frequency
  2. Dysuria (painful burning sensation on urination)
  3. Hematuria (blood in urine)
  4. Suprapubic pain along with loin pain and tenderness
  5. Fevers and chills

LABORATORY DIAGNOSIS

A) Specimen Collection:

1. Midstream clean-catch technique/MSU (commonest method):

  • Simple, inexpensive and noninvasive
  • Cleansing of skin and mucous membrane adjacent to the urethral orifice before urination
  • First part of urination is allowed to pass into the toilet
  • Mid-portion of the stream is then collected in sterile container
  • Disadvantage: Despite precautions, contamination of urine can occur

2. Straight catheter technique:

  • Urine should be collected directly from the catheter and not from the collection bag
  • Catheter should not be allowed to touch the container
  • Disadvantage: Labor intensive, costly and invasive, risk of UTI

3. Suprapubic aspiration (rarely used):

  • Best method to avoid contamination of specimen
  • Disadvantage: Invasive, costly, time consuming, requires too much resources

B) Storage or Transport:

In case of delay in specimen processing (>2 hours), either of the following must be done:

  1. Storage: Refrigeration (4c) or Preservatives (Boric acid)
  2. Transport using Dip-slide method: Discussed below

C) Specimen processing:

1. Microscopic examination:

  • Pyuria (Leukocytes in urinary sediment): >10/hpf is indicative of UTI
  • Hematuria (RBCs and casts)
  • Bacteriuria (Bacteria in uncentrifuged urine by gram stain): ≥1/hpf is indicative of UTI

2. Culture:

Routine culture media: Blood agar and MacConkey’s agar

Semi-quantitative culture:

  1. Standard loop technique: Calibrated loop is used to hold certain volume of urine which is cultured and incubated at 37c for 18-24 hours except in suspected funguria for which incubation is done for 48 hours to count colonies. Eg. If a loop can hold 0.004 ml of urine, then 250 loopfuls make 1ml. Hence colony count is expressed per 1ml by: 250 X Number of colonies in 1 loop
  2. Dip slides technique: Plastic slides coated with CLED (Cysteine lactose electrolyte deficient) agar on one side and MacConkey’s agar on the other side is dipped into freshly voided urine and replaced in a sterile container and incubated as in standard loop method. Viable count is obtaine by comparing growth on the media with the manufacturer’s chart.

Interpretation of results for different specimens:

1. MSU:

  • ≥10^5 CFU/ml (Indicates UTI and sensitivity test is done)
  • 10^3 to 10^5 CFU/ml (Indicates doubtful of UTI/contaminated and culture is repeated)
  • ≤10^3 CFU/ml (Indicates contamination)

2. Suprapubic aspiration and Catheter: ≥10^2 CFU/ml (Indicates UTI)

D) Identification of organism:

a) Urine Biochemistry:

  1. Proteinuria
  2. Blood
  3. Nitrite (Nitrate reduction) test: In Enterobacteriacea associated UTI
  4. Leukocyte esterase test

b) Antimicrobial susceptibility testing:

3 Comments
  1. October 17, 2011
    • October 19, 2011
  2. September 23, 2013

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