Case 5

Urine bacterial culture and antimicrobial sensitivity

Organism: Pseudomonas species > 100,000 cfu/ml

Antibiotic (µg/ml)

MIC

Interpretation

Reference range

Amikacin

≤ 8

S

8-64

Amoxicillin/clavulanate

NA

R

4-32

Ampicillin/amoxicillin

NA

R

0.25-32

Cefpodoxime

≥ 8

R

2-8

Cephalexin

NA

R

4-32

Enrofloxacin

2

I

0.5-4

Marbofloxacin

1

S

0.5-4

Nitrofurantoin

> 128

R

16-128

Trimethoprim/sulfonamide

NA

R

0.5-4

Ticarcillin

≤16

S

16-128

NA = not applicable

S = susceptible, R= resistant, I = intermediate

The urine bacterial culture results indicate that Coco has developed a resistant UTI.

Interpretation of MIC data

The minimal inhibitory concentration (MIC) is the lowest amount (dilution) of antibiotic that inhibits bacterial growth. In order to be effective, the antibiotic you choose must reach this MIC at the site of the infection. However, in vitro susceptibility testing may not translate to in vivo efficacy since the MIC data is based on serum antibiotic concentrations and does not account for patient or drug factors. For example, for a UTI, you would want to choose a drug that concentrates well in the urine (e.g. amoxicillin), whereas for treatment of a patient with pneumonia, you would want to consider a drug that concentrates well in WBCs (e.g. fluoroquinolones).

Obviously, you also want to choose a drug that demonstrates a MIC below the susceptible breakpoint (below the lowest end of the reference range provided). Organisms with antibiotic MICs above the high end of the breakpoint range are considered resistant to that antibiotic, and those with MICs within the reference range are typically considered to have intermediate sensitivity to that antibiotic. In some cases, drugs classified as intermediate may still be considered for treating the infection depending on the site of the infection and the drug. For example, a drug that demonstrates intermediate sensitivity may still be an appropriate choice if it can be used topically at the site of the infection, such as in patients with otitis.

QUESTION 3

What is your new treatment plan for Coco at this time?

a) Recommend treatment with amikacin and monitor her renal function parameters closely.

b) Dispense marbofloxacin for three weeks and recommend that Coco return for a recheck urinalysis in one week.

c) Recommend an extended sensitivity panel to identify another antibiotic that may be used at this time.

d) Recommend an abdominal ultrasonographic examination to look for bladder abnormalities that may be predisposing Coco to an infection.

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