Sunday, March 4, 2018

UTI Series: Pathogenesis, risk factors and diagnosis

Hello Awesomites! :)

This post on Urinary Tract Infection (UTI) is brought to you by our passionate MSGA Calvin Ong K.Y. and me, Upasana Y. 

The following parts can be infected in an UTI:
- Kidney
- Urinary bladder
- Ureter

Infections of urethra is known as Urethritis, which is dealt under different clinical syndromes. Infection of the urethra is mainly caused by N. gonorrhoeae, C. trachomatis, M. genitalium. T. vaginalis, HSV, and adenovirus can also cause urethritis.


Escherichia coli - It is the most common urinary pathogen.

Proteus, Klebsiella, Pseudomonas species and Staphylococcus aureus are associated with hospital acquired infections because their resistance to antibiotics favor their selection. Catheterization and gynecological surgery increase risk for these infections.

Proteus infections are associated with renal stones. Proteus produces a potent urease which acts on ammonia, rendering the urine alkaline.

S. saprophyticus infections are found in sexually active young women.

Candida infection is usually seen in diabetic patients and in the immunosuppressed.

M. tuberculosis is carried in blood to kidney from another site of infection. (eg. respiratory TB)

Polymicrobial bacteruria is due to fistulas, urinary retention, infected stones or catheters.

Pathogenesis of UTI

1. COLONIZATION - Pathogens colonizes the periurethral area and ascends through urethra upward towards the bladder.

2. UROEPITHELIUM PENETRATION - Fimbria allow bladder epithelial cell attachment and penetration. Bacteria continue to replicate and may form biofilm.

3. ASCENSION  -Bacterial toxins may also play a role by inhibiting peristalsis (reducing the flow of urine)



Risk factors of UTI

  • Indwelling catheter
  • Antibiotic use
  • Spermicides
  • Voiding dysfunction
  • Frequent or recurrent sexual intercourse
  • Vesicoureteral reflux
  • Female sex (short urethra ~4cm)
  • Pregnancy (progesterone mediated smooth muscle relaxation to the bladder and ureters and compression of ureters by the uterus)
  • Familial tendency
  • Susceptible uroepithelial cells 
  • Vaginal mucus properties
Route of spread
  • Ascending route
  • Hematogenous
  • Lymphatic
Signs & Symptoms of Urinary Tract Infection
Urinary tract affected:
1. Urethra – cause urethritis
-Burning and pain with urination 
(Urethritis is classified as an STI and not UTI by many textbooks)

2. Bladder – cause cystitis
-Painful urination
-Frequent and persistent urge to urinate
-Lower abdomen discomfort
-Cloudy/Strong-smelling urine

3. Kidneys – cause pyelonephritis
-Flank pain (Upper back and side)
-Nausea and vomiting

Diagnostic Testing for Urinary Tract Infections:

Types of urine Samples
-Mid stream Urine sample
-Catheter specimen of urine during cystoscopy
-Suprapubic aspirate
-Early morning urine (TB of urinary tract)
-Initial flow (Urethritis, prostatitis)
1. Urine microscopy
-Pyuria (pus presented in urine + elevated white blood cells in urine)
-Hematuria (red blood cells in urine)
RBCs may be found in the urine during menstruation in a woman’s urine sample, thus leading to a false positive result.
-Motile bacteria – E.Coli, Proteus, Pseudomonas
-Non-motile bacteria – Klebsiella
-Presence of cocci – Staphylo-, Strepto-, Enterococci 
**Presence of WBC casts indicates pyelonephristis rather than cystitis
**If urine sample contains abundant squamous epithelial cells - sample is contaminated and results are not reliable

2. Urine dipsticks 
-Use different chemicals reagants on a strip that is dipped in urine to diagnose urinary tract diseases
-E.g. of dipstick result (positive leukocyte esterase, positive nitrite, positive haemoglobin)

3. Urine culture
-Culture of mid-stream urine – Blood agar, Mac Conkey agar
-Midstream void - ≥1 × 105 CFU/μL is consistent with infection
-Samples collected via catheterization -≥1 × 102 CFU/μL is consistent with infection
**Contamination of samples may occur when urine passes through outer third of urethra
**Therefore, numeric threshold of colony-forming units (CFU) per millilitre is used to confirm infection.

4. Imaging test:- 
It is not routinely done in case of UTI.

Ultrasonography is indicated
-Obstruction in urinary flow
-Measurement of bladder residual volume in BPH 
-Recurrent UTI 
-Pyelonephritis or hematuria.

KUB is most useful in suspected case of urolithiasis.

Computed tomography urography is used to view the kidneys and adjacent structures, and may be considered to further evaluate patients with possible abscess, obstruction, or suspected anomalies when ultrasonography is not diagnostic.

If urinalysis is unrevealing, cystoscopy can be performed to evaluate for bladder cancer, hematuria, and chronic bladder symptoms.

Urodynamic studies can be performed for persistent voiding symptoms.

Intravenous urography - for hematuria evaluation if CT urography is not available.

Men with UTI
US with abdominal X-RAY and flow rate
  • No abnormality detected  - no further imaging 
  • Abnormal upper tract 
  • Abnormal lower urinary tract - further investigation (e.g.cystoscopy,urodynamics or transrectal US)
We are grateful to our teachers. :)

- Upasana Y. and Calvin Ong K.Y.


  1. Very useful concepts regarding various pathogens .......
    Thank you , seriously it's very useful one .
    Thank u Upasna and Calvin

  2. Hey! Thank you so much. We are glad it helped. :)


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