Results

Over the 5-year period (2014–2019), 25 (6%) of 416 of adults with MRSA BSI had an isolate with a vancomycin MIC ≥ 2 µg/mL. Overall, 9 patients had an MIC ≥ 2 µg/mL on initial cultures. Among them, 4 patients were transferred from other hospitals; thus, culture information was not available. All 25 cases were previously exposed to vancomycin.

Cases had higher odds of baseline hemodialysis (odds ratio [OR], 3.61; 95% confidence interval [CI], 1.55–8.40; P = .003) and a history of MRSA colonization (OR, 5.87; 95% CI, 2.29–15.05; P < .001) (Table 1). Although only approaching significance, cases had higher odds of hospital admission in the prior 90 days (OR, 2.80; 95% CI, 0.94–8.32; P = .06). Ability to achieve source control did not differ between the groups. The frequency of elevated vancomycin MIC did not increase over the study period.

Table 1.

Univariable Analysis of Demographics and Clinical Characteristics of Cases with MRSA BSI with MIC ≥ 2 µg/mL Compared to Controls

Note. BSI, bloodstream infection; HIV, human immunodeficiency virus; NH, nursing home; LTACH, long-term acute care hospital; MRSA, methicillin-resistant Staphylococcus aureus. CO, community-onset; HO, hospital-onset; IQR, interquartile range; ICU, intensive care unit; MIC, minimum inhibitory concentration.

^a Included nonmedical residences such as home, group homes, assisted living facilities, and homeless shelters.

^b Included pacemaker, implantable cardioverter defibrillator (ICD), left ventricular-assist device (LVAD), vascular access (excluding peripheral intravenous catheters), orthopedic hardware, nephrostomy, suprapubic catheter, ileal conduit, foley catheter, arteriovenous graft placement (AVG), percutaneous endoscopic gastrostomy (PEG) tube, or ostomy.

^c Included any invasive procedures or surgery occurring within 1 month before first positive blood culture for MRSA, excluding electroencephalogram (EEG), electrocardiogram (EKG), and transthoracic echocardiogram (TTE).

^d Included presence of a chronic skin wound overlying the sacrum, limb, abdomen, or other body part.

^e Refer to standard definitions for CCI.

^f Included solid organ and bone marrow transplant.

^g Any positive culture from urine, sputum, tissue, or nares with MRSA prior to the positive MRSA blood culture or a documented history of prior MRSA infection or colonization.

^h MRSA infection from urinary source, osteomyelitis, surgical site infection, spinal infection, septic arthritis or cardiac device infection, or an unknown/not reported source.

ⁱ Measured as time to development of elevated vancomycin MIC for cases and time to clearance of BSI in controls.

An outcome analysis revealed a 2.62 higher odds of recurrent MRSA BSI in cases (95% CI, 0.99–6.94; P = .05) (Table 2). Cases also had a 2.86 (95% CI, 1.23–6.61) higher odds of developing metastatic infection during their index admission for BSI (P = .01). Median time to discharge after initial positivity reached borderline significance at 34 days for cases (IQR, 15–47) compared with 15 days (IQR, 8–24) for controls (95% CI, 0.97–2.49; P = .07). Mortality rates did not differ.

Table 2.

Outcomes of Cases with MRSA BSI with MIC ≥ 2 µg/mL Compared With Controls

Note. MRSA, methicillin-resistant Staphylococcus aureus; BSI, bloodstream infection; ICU, intensive care unit; IQR, interquartile range

^a Excluding patients on dialysis

^b Positive blood culture over 30 days after the last positive blood culture

^c Distal or secondary infection, anatomically unrelated to the primary site of infection, presenting during index admission of MRSA BSI. Includes infective endocarditis, septic pulmonary emboli, vertebral osteomyelitis, septic arthritis, iliopsoas abscess.

All 25 cases were initially treated with vancomycin monotherapy, and 7 patients (28%) completed treatment with vancomycin despite the elevated MIC because clinical response was achieved. For the remaining 18 patients (72%) who were switched from vancomycin, several reasons were documented: increasing MIC (n = 4), failure to clear cultures (n = 5), synergy of combination therapy (n = 5), persistent fevers (n = 1), difficulty dosing (n = 1), and lost parenteral access (n = 2). In 4 cases, regimens were adjusted due to detection of heterogeneous vancomycin-intermediate S. aureus (hVISA), but hVISA testing was stopped in 2016 and no longer influenced decision making.

Antibiotics utilized for the 18 cases (72%) that were switched from vancomycin therapy varied. Monotherapy was adjusted to daptomycin (n = 10), linezolid (n = 4), or telavancin (n = 1). Combination regimens included use of β-lactams (daptomycin-nafcillin, daptomycin-cefazolin, vancomycin-cefazolin, linezolid-cefazolin), and dual MRSA coverage (vancomycin-trimethoprim–sulfamethoxazole, vancomycin-daptomycin, linezolid-ceftaroline, vancomycin-ceftaroline, daptomycin-ceftaroline). Others included linezolid-tigecycline, and vancomycin-gentamicin. Adjunctive rifampin was used in 4 cases to clear bacteremia and reduce biofilm in disseminated infections. Three cases died or eloped prior to MIC elevation, and adjustments were not applicable.