December 6, 2023

Urinary Tract Infection (UTI) : Etiopathogenesis and Lab Diagnosis

  • October 5, 2011
  • 4 min read
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



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


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:

About Author

Sulabh Shrestha

Intern doctor and Medical Blogger Sulabh Shrestha


  • Thanks for letting us know. It was useful. However, this can also spread through unhygienic public toilets. We must take care while using public toilets & use some highly effective & safe toilet seat sanitizer sprays available in the market. Or we can also make use of disposable toilet seat cover.

  • it was helpful !
    thanx a lot ~

    • Nice to know that it helped you

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