Bloodstream Infections and Infective Endocarditis Study Notes

Bloodstream Infections (BSI) Classification & Diagnostics

• Epidemiology & Significance:

  • Gram-negative BSI accounts for approximately $250,000$ patients per year.

  • Staphylococcus aureus bacteremia carries a mortality rate of $20\text{--}40\%$.

  • Catheter-associated BSIs (CLABSI) are common, with $24,265$ cases recorded in 2017.

  • Viremia is predominantly seen in severely immunocompromised individuals and is not captured via routine blood cultures.

• Contamination vs. True Pathogens:

  • Common Contaminants (Skin Flora): Coagulase-negative Staphylococci (CoNS), Corynebacterium spp. (diphtheroids), Bacillus spp. (excluding B. anthracis), and Micrococcus spp.

  • Factors Suggesting Contamination: Low number of positive bottles (e.g., $1$ of $4$), longer time to positivity ($> 24\text{ hours}$), hemodynamic stability, and absence of implanted hardware.

  • Pathogens NEVER Considered Contaminants: Staphylococcus aureus, Enterococcus spp., Gram-negative bacteria, Bacteroides fragilis, and fungi (e.g., Candida albicans).

• Catheter & Line-Associated Sources:

  • Foreign hardware (Central Venous Cathetres [CVC], PICCs, implanted ports) poses high infection/colonization risks.

  • CLABSI diagnosis requires matching blood cultures from CVC and peripheral lines (or catheter tip positive with peripheral blood) in the absence of another primary source.

BSI Empiric Therapy & Treatment Duration

• Empiric Antimicrobial Regimens:

  • Gram-Positive Coverage: Vancomycin or Daptomycin.

  • Gram-Negative Bacilli Coverage: Cefepime, Piperacillin-tazobactam, or Meropenem/other carbapenems.

  • Fungal Coverage: Echinocandins or Fluconazole (if not used within $> 3\text{ months}$ and risk of Candida krusei or Candida glabrata is low).

  • High-Risk Specific Additions: Add antipseudomonal agents for neutropenia, sepsis, or known Pseudomonas colonization. Add empiric antifungal coverage for TPN, broad-spectrum antibiotics, bone marrow/solid organ transplant, hematologic malignancy, or femoral catheters.

• BSI Treatment Duration Guidelines:

  • Day 1 of therapy is defined as the first negative blood culture (or day 1 of effective antibiotic if no repeat culture is drawn).

• Monitoring & Oral Transition Criteria:

  • Repeat Cultures: Perform every $24\text{--}48\text{ hours}$ for S. aureus, S. lugdunensis, and Candida spp. until clear. Gram-negative and other Gram-positive BSIs with source control and clinical improvement do not require repeat cultures.

  • Oral Antibiotic Transition in Uncomplicated Gram-Negative BSI:

  • Uncomplicated criteria: Secondary to 1 of 5 main sources, source control achieved, non-immunocompromised, and clinical improvement within $72\text{ hours}$.

  • Duration: Total course of $7\text{ days}$.

  • Preferred oral agents: Fluoroquinolones or Trimethoprim-sulfamethoxazole (TMP-SMX); highly bioavailable oral beta-lactams (e.g., Cefadroxil) can be considered. Avoid Fosfomycin and Nitrofurantoin due to poor systemic absorption.

Infective Endocarditis (IE) Diagnosis & Criteria

• Risk Factors & Pathophysiology:

  • High-risk states: Prosthetic heart valves, structural/valvular dysfunction, congenital heart disease, prior IE, persons who inject drugs (PWID), and poor dental hygiene.

  • Disease sequence: Endocardial injury $\rightarrow$ Bacteremia $\rightarrow$ Adherence to activated platelets $\rightarrow$ Proliferation and vegetation formation.

• Clinical Manifestations:

  • General: Fever, night sweats, leukocytosis, headache, arthralgias, cardiac murmur.

  • Vascular/Immunologic Phenomena: Janeway lesions, petechiae, splinter hemorrhages, Osler nodes, and Roth spots.

• Echocardiography:

  • Transthoracic Echocardiogram (TTE): Non-invasive with high specificity, but lower sensitivity.

  • Transesophageal Echocardiogram (TEE): High sensitivity, required for evaluating prosthetic valve endocarditis (PVE).

• Modified Duke Criteria:

  • Major Criteria:

  • Blood cultures positive for typical IE pathogens (Viridans group Streptococci, Streptococcus gallolyticus, HACEK group, S. aureus, Enterococcus faecalis) in $2$ separate blood samples.

  • Single positive blood culture or IgG antibody titer $\ge 1:800$ for Coxiella burnetii.

  • Echocardiogram, CT, or PET positive for IE (vegetation, abscess, or valve dehiscence).

  • Minor Criteria:

  • Predisposition (PWID, prosthetic valve, cardiac hardware, pre-existing valve dysfunction).

  • Temperature $> 38^\circ\text{C}$.

  • Vascular phenomena (major arterial emboli, septic pulmonary infarcts, Janeway lesions, conjunctival hemorrhages).

  • Immunologic phenomena (glomerulonephritis, Osler nodes, Roth spots, rheumatoid factor).

  • Positive blood culture/PCR not meeting major criteria.

  • Diagnostic Thresholds:

Pathogen-Directed Treatment for Infective Endocarditis

• Streptococcal Endocarditis:

  • Native Valve (NVE), Highly Penicillin Susceptible (MIC $\le 0.12\,\text{mg/L}$):

  • Penicillin G or Ceftriaxone for $4\text{ weeks}$ (or $2\text{ weeks}$ if combined with Gentamicin).

  • Penicillin allergy: Vancomycin for $4\text{ weeks}$.

  • Prosthetic Valve (PVE), Highly Penicillin Susceptible: Penicillin G or Ceftriaxone for $6\text{ weeks}$ $\pm$ Gentamicin for the first $2\text{ weeks}$.

• Staphylococcal Endocarditis:

  • Native Valve (NVE - MSSA): Nafcillin or Oxacillin for $6\text{ weeks}$ ($2\text{ weeks}$ for uncomplicated right-sided IE). Non-anaphylactic penicillin allergy: Cefazolin for $6\text{ weeks}$.

  • Native Valve (NVE - MRSA / Severe Penicillin Allergy): Vancomycin or Daptomycin for $6\text{ weeks}$.

  • Prosthetic Valve (PVE - MSSA): Nafcillin or Oxacillin for $\ge 6\text{ weeks}$ + Rifampin for $\ge 6\text{ weeks}$ + Gentamicin for $2\text{ weeks}$.

  • Prosthetic Valve (PVE - MRSA / Penicillin Allergy): Vancomycin for $\ge 6\text{ weeks}$ + Rifampin for $\ge 6\text{ weeks}$ + Gentamicin for $2\text{ weeks}$.

• Enterococcal Endocarditis:

  • Penicillin & Gentamicin Susceptible: Ampicillin or Penicillin G + Gentamicin for $4\text{--}6\text{ weeks}$, OR Dual Beta-Lactam therapy (Ampicillin + Ceftriaxone) for $6\text{ weeks}$.

  • Penicillin Resistant / Allergy: Vancomycin + Gentamicin for $6\text{ weeks}$.

Practice Cases & Applications

• Case 1: Septic Patient with CVC Growth:

  • Presentation: Patient spiked fever to $102.3^\circ\text{F}$, HR $124$, RR $33$, lactic acid $3.4\,\text{mg/dL}$, WBC $18 \times 10^9/\text{L}$. Growth of non-lactose fermenting Gram-negative rod in $2$ of $4$ bottles after CVC placement $1\text{ week}$ prior.

  • Management: Gram-negative bacilli are never contaminants. Non-lactose fermenters suggest Pseudomonas aeruginosa or Acinetobacter baumannii. Ceftriaxone lacks Pseudomonas coverage; switch to an antipseudomonal agent (e.g., Cefepime, Piperacillin-tazobactam). Remove CVC as source control. Treat for $7\text{--}14\text{ days}$ post-line removal.

• Case 2: Determining Treatment Necessity for Positive Cultures:

  • Scenario A: CoNS in $1$ of $4$ bottles at $36\text{ hours}$ $\rightarrow$ Contaminant, do not treat.

  • Scenario B: MSSA in $1$ of $4$ bottles at $24\text{ hours}$ $\rightarrow$ Pathogen (never a contaminant), treat.

  • Scenario C: Candida albicans in $1$ of $4$ bottles at $48\text{ hours}$ $\rightarrow$ Pathogen (never a contaminant), treat.

  • Scenario D: Corynebacterium spp. in $4$ of $4$ bottles at $12\text{ hours}$ $\rightarrow$ High yield/true infection despite skin flora classification, treat.

• Case 3: PWID with MSSA Endocarditis & Non-Anaphylactic Penicillin Allergy:

  • Presentation: PWID with MSSA bacteremia, TEE positive for large vegetation. Amoxicillin allergy history of shortness of breath and dyspepsia $3\text{ years}$ ago.

  • Management: Patient has non-anaphylactic allergy; cephalosporins are safe. Select Cefazolin $2\,\text{g}$ IV every $8\text{ hours}$ for $4\text{--}6\text{ weeks}$ from the first negative blood culture. Discontinue initial empiric Vancomycin, Cefepime, and Metronidazole.