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Diagnosis of UTI: The culture of “Urine Cultures”

MIC testing plays a critical role in reducing the burden of antibiotic resistance

UTI has a high prevalence in India and the world over in every age group
Global data published in 2016 estimates the frequency of UTIs among hospital-acquired infections to range from 13 to 19% in the US and Europe to as high as 25% in developing countries
In the US alone, 40% of women and 12% of men have at least one episode of UTI in their lifetime
UTIs is the most common medical complications of pregnancy together with anaemia and hypertension and it occurs approximately 5% – 10% of all pregnancies

However, there seems to be lack of nation-wide consolidated data regarding the epidemiology of UTI in India. Hence, at Suburban Diagnostics, we made an attempt to try and bridge this gap and take the first step to fulfil this unmet need. We conducted a retrospective study on the epidemiological patterns and antibiotic susceptibility patterns prevailing among 24,000 urine cultures done at our lab.

At Suburban Diagnostics, we conducted a retrospective analysis of approximately 25,000 urine culture results for community-acquired (CA) UTI over the last 4 years.

GENDER AND AGE DISTRIBUTION

Females predominate community-acquired (CA) UTI (71%)

Advancing age came across very strongly as a risk factor for UTI. We noted an almost 20% sharp rise in the prevalence of UTI, as advancing from the age group of 40-60 years to 60 and above.

The major chunk of causative pathogens was Gram-negative and of these the most commonly reported bacteria was E.coli.

ORGANISM DISTRIBUTION

Almost 60% of isolates were ESBL positive which clinically represents a more aggressive and resistant form of infection.

The urine culture report also provides the clinicians with an antibiotic susceptibility analysis detailing which are the most effective antibiotics for a particular case of UTI. This varies for every patient and helps the clinician in selecting the most effective drugs and can go a long way in improving prognosis.

Let’s understand how this antibiotic susceptibility is decided.

The Role of MIC & Breakpoint MIC in controlling Antibiotic Resistance

MIC (Minimum Inhibitory Concentration) is the lowest concentration of an anti microbial agent that prevents the visible growth of a bacterium
MIC is a measure of anti microbial potency. Using the MICs, a tested strain can then be categorized according to its susceptibility for the antimicrobial agent tested
The strain is said to be Susceptible(S), Intermediate (I), or Resistant (R) to the antibacterial or antifungal agent
A breakpoint MIC is a chosen concentration (mg/L) of an antibiotic which defines whether a species of bacteria is susceptible or resistant to the antibiotic. If the MIC is less than or equal to the breakpoint MIC, the bacteria is considered susceptible to the antibiotic

The antibiotic susceptibility pattern noted in our study has been shown below:

An example of how breakpoint MICs helps us decide the most appropriate antibiotic out of the ones that are sensitive.

Antibiotic	Interpretation	MIC Values	Break Point MICs
Cefixime	Sensitive	2	4
Amoxicillin – clavulanic acid	Sensitive	0.5	16
Nitrofurantoin	Sensitive	16	64
Norfloxacin	Sensitive	4	16

The antibiotic which shows the highest “breakpoint MIC-to-MIC” ratio is the most sensitive antibiotic and hence is the drug of choice.

Amoxycillin Clavulanic acid is the most appropriate antibiotic to be used in the above example.