Case Exercises
By simulating real world antibiograms and clinical cases, Case Exercises help reinforce scientific principles explained and demonstrated in Modules and translate antibiogram data into clinical decision-making. Please select the case to review.
Case 3. MRSA in the Community
A 16-year-old high school football player was brought to the Emergency Department by his mother who noticed that a two-day-old abrasion on his left elbow had now become severely inflamed and purulent and his entire arm was swollen. Two weeks earlier the school had sent out a notice informing parents that several members of the football team were diagnosed with MRSA infections and the public health department was investigating a potential outbreak.
This Hospital’s Annual Antibiogram for Staphylococcus aureus
| Organism |
Location |
n |
% Susceptible |
| Oxacillin |
Ciprofloxacin |
Clindamycin |
Daptomycin |
Doxycycline |
Erythromycin |
Linezolid |
Rifampin |
Trimethoprim/ Sulfamethoxazole |
Vancomycin |
| S. aureus |
All |
1582 |
62 |
63 |
72 |
99 |
98 |
51 |
99 |
98 |
98 |
99 |
| MRSA |
OP |
343 |
0 |
20 |
71 |
100 |
96 |
0 |
100 |
98 |
98 |
100 |
| IP |
106 |
0 |
14 |
45 |
99 |
98 |
0 |
100 |
99 |
97 |
99 |
| ICU |
152 |
0 |
8 |
42 |
99 |
96 |
0 |
99 |
99 |
97 |
100 |
| MSSA |
OP |
668 |
100 |
85 |
77 |
100 |
99 |
64 |
100 |
99 |
99 |
100 |
| IP |
151 |
100 |
83 |
75 |
100 |
98 |
66 |
100 |
99 |
99 |
100 |
| ICU |
162 |
100 |
81 |
73 |
100 |
99 |
69 |
100 |
98 |
98 |
100 |
OP, outpatient; IP, inpatient excluding ICU patients; ICU, ICU patients.
CLINICAL CONSIDERATIONS
Based on the patient’s symptoms and history, the attending physician who was new to this facility suspected MRSA. Due to the boy’s age and allergy to sulfa medications, oral therapy options were limited. The physician contemplated using clindamycin but wanted to know the incidence of clindamycin susceptibility in this community. What percentage is most representative of clindamycin susceptibility among MRSA in the community?
Answer & Rationale
How can the physician be certain that the clindamycin % Susceptible statistics took into consideration isolates with “inducible clindamycin resistance”?
- All laboratories always test for inducible clindamycin resistance among MRSA so he can assume this was done.
- Since special tests are required for detecting inducible clindamycin resistance, it would be best to contact the laboratory to determine if they are doing such testing routinely and factoring results from this testing into their antibiogram.
- Since the % Susceptible for erythromycin is lower than the % Susceptible for clindamycin, he can assume this was done.
- None of the above
Answer & Rationale
Which statistic most closely represents the number of patients with MRSA seen at this facility?
- 981 (62% of 1582) – includes first isolate/patient only for 1582 patients with S. aureus
- 601 – total of OP + IP + ICU patients with MRSA
- 388 – OP + IP with MRSA since IP includes ICU patients
- Unable to determine from these data because they include repeat MRSA isolates from multiple cultures positive for MRSA for many patients
Answer & Rationale
How many patients at this institution had an isolate of daptomycin-nonsusceptible MRSA?
- 1% (100%-99%) of 1582 isolates = 16 patients
- 1% of (106 + 92) = 2 patients
- 16 + 2 = 16 patients
- Cannot determine from these data
Answer & Rationale
The correct answer is 72%.
The correct answer can be found in the % Susceptible to clindamycin for MRSA isolates collected from outpatients.
The correct answer is Since special tests are required for detecting inducible clindamycin resistance, it would be best to contact the laboratory to determine if they are doing such testing routinely and factoring results from this testing into their antibiogram.
The correct answer is 601.
It is important to remember that antibiograms should include only the first isolate of a specific organism per patient per time period (therefore the last answer choice is incorrect). The answer choice 981 is an estimate of patients with MSSA isolates since 62% is the susceptibility rate to oxacillin (rather than the resistance rate). The antibiogram also states that IP data does not include ICU patients, so the third answer choice is incorrect. The correct answer is the sum of MRSA isolates collected from OP, IP, and ICU patients.
The correct answer is Cannot determine from these data.
The antibiogram includes only first isolates per patient; therefore, the report will not include any patient who may have had a subsequent isolate collected that exhibits resistance to daptomycin. Resistance to potent agents such as daptomycin can occur following prolonged hospitalization and/or antimicrobial therapy. To increase awareness of the potential for resistance to potent agents at an institution, it may be helpful to include the total number of resistant isolates collected in the antibiogram report.