ABIM Infectious Disease cheat sheet

Beta-lactam mechanism: acylation of penicillin-binding proteins (transpeptidases), blocking peptidoglycan cross-linking; resistance via beta-lactamase hydrolysis, altered PBPs (MRSA mecA/PBP2a), or efflux.

Aminoglycosides bind the 30S subunit and cause mRNA misreading; they are concentration-dependent killers with a post-antibiotic effect, so extended-interval dosing is preferred except in endocarditis and pregnancy.

Vancomycin targets lipid II D-Ala-D-Ala; constitutive resistance requires vanA/vanB D-Ala-D-Lac substitution (VRE). AUC/MIC 400-600 is the target for serious MRSA infection.

Fluoroquinolones inhibit DNA gyrase (topoisomerase II) and topoisomerase IV; they are concentration-dependent, time-dependent killers with minimal post-antibiotic effect.

Macrolides and clindamycin bind the 50S subunit; macrolide resistance via erm methylation (MLS-B phenotype) or mef efflux.

For community-acquired pneumonia, standard empiric therapy is a beta-lactam plus macrolide or a respiratory fluoroquinolone; MRSA and Pseudomonas coverage are added only with validated risk factors.

Infective endocarditis: native valve empiric therapy is vancomycin plus ceftriaxone; prosthetic valve adds gentamicin and rifampin for staphylococcal coverage.

Clostridioides difficile: oral vancomycin or fidaxomicin are first-line; metronidazole is reserved for when preferred agents are unavailable.

Necrotizing fasciitis: immediate surgical debridement is the primary intervention; antibiotics are adjunctive and do not substitute for source control.

Influenza: neuraminidase inhibitors (oseltamivir, zanamivir) are most effective within 48 h of symptom onset; treatment is still recommended for hospitalized or high-risk patients beyond 48 h.

HSV/VZV: acyclovir is activated by viral thymidine kinase; resistance arises from thymidine kinase or DNA polymerase mutations, most often in immunocompromised hosts.

CMV: ganciclovir requires phosphorylation by UL97 kinase; UL97 mutations cause ganciclovir resistance, while UL54 mutations affect DNA polymerase and may confer cross-resistance to cidofovir and foscarnet.

Hepatitis B: nucleos(t)ide analogues (tenofovir, entecavir) suppress but rarely eradicate; HBsAg loss is the functional cure endpoint and is uncommon.

Hepatitis C: direct-acting antivirals (NS5B polymerase, NS5A, NS3/4A protease inhibitors) achieve SVR12 in >95% of treated patients across genotypes.

RSV: supportive care is standard in immunocompetent adults; ribavirin is used in select immunocompromised or high-risk patients, with limited evidence.

Measles: no antiviral; post-exposure prophylaxis is MMR vaccine within 72 h or immunoglobulin within 6 days for susceptible contacts.

Varicella zoster: post-exposure prophylaxis with VZIG or acyclovir is indicated for susceptible immunocompromised contacts.

Amphotericin B binds ergosterol, forming pores; it is nephrotoxic and causes infusion reactions; liposomal formulations reduce toxicity and are preferred for invasive mold and mucormycosis.

Azoles inhibit 14-alpha-demethylase (CYP51/ERG11); resistance via ERG11 mutations or efflux pumps. Voriconazole is first-line for invasive aspergillosis.

Echinocandins inhibit beta-1,3-D-glucan synthase; they are fungicidal against Candida and fungistatic against Aspergillus, and are first-line for candidemia.

Cryptococcal meningitis: induction with amphotericin B plus flucytosine for at least 2 weeks, then fluconazole consolidation; screen and treat for elevated intracranial pressure.

Mucormycosis: high-dose liposomal amphotericin B plus urgent surgical debridement; avoid voriconazole (inactive against Mucorales).

Invasive aspergillosis: voriconazole or isavuconazole first-line; serum galactomannan and beta-D-glucan are supporting diagnostic markers.

Candida auris: often multidrug-resistant; use echinocandin first-line, and perform contact precautions and environmental disinfection.

Pneumocystis jirovecii: trimethoprim-sulfamethoxazole first-line for treatment and prophylaxis; alternatives include atovaquone, pentamidine, and clindamycin-primaquine.

Latent TB: 4R (4 months rifampin), 3HP (3 months weekly isoniazid plus rifapentine), or 9H are standard regimens; rule out active disease before treating latent infection.

Active drug-susceptible TB: 2 months of isoniazid, rifampin, pyrazinamide, and ethambutol, then 4 months of isoniazid plus rifampin (2HRZE/4HR).

Isoniazid hepatotoxicity and peripheral neuropathy (prevent with pyridoxine); rifampin induces CYP450 and causes orange body fluids; ethambutol causes optic neuritis; pyrazinamide causes hyperuricemia.

MDR-TB: resistance to at least isoniazid and rifampin; treat with a fluoroquinolone plus bedaquiline, linezolid, and other agents per WHO grouping.

Mycobacterium avium complex: azithromycin plus ethambutol, with or without rifabutin; indicated for disseminated disease in advanced HIV (CD4 <50).

Nontuberculous mycobacteria: species identification and susceptibility testing guide therapy; M. abscessus is often multidrug-resistant and requires a macrolide-based regimen when susceptible.

Tuberculin skin test and interferon-gamma release assays detect infection, not active disease; IGRA is not affected by BCG vaccination.

Paradoxical reaction / IRIS in TB: transient worsening of symptoms or lesions after starting therapy or immune reconstitution; treat with continued therapy and anti-inflammatory measures.

Malaria: Plasmodium falciparum requires artemisinin-based combination therapy; P. vivax/ovale require a primaquine or tafenoquine course to eradicate hypnozoites (check G6PD first).

Severe malaria: intravenous artesunate is first-line; exchange transfusion is not routinely recommended.

Babesiosis: azithromycin plus atovaquone for mild disease; clindamycin plus quinine for severe disease; consider in asplenic or post-transplant patients with fever and hemolysis.

Toxoplasmosis: pyrimethamine plus sulfadiazine plus folinic acid; in pregnancy, spiramycin is used to prevent fetal transmission.

Leishmaniasis: liposomal amphotericin B is preferred for visceral disease; cutaneous disease may be treated with local therapy or systemic agents depending on species.

Strongyloidiasis: ivermectin is first-line; hyperinfection syndrome occurs with corticosteroid use or HTLV-1 and requires prolonged therapy.

Chagas disease: benznidazole or nifurtimox; screening of blood donors and transplant recipients is important in endemic regions.

Giardiasis: tinidazole or metronidazole; consider nitazoxanide for refractory cases.

Hand hygiene is the single most effective measure to prevent healthcare-associated infection; alcohol-based hand rub is preferred except for C. difficile (use soap and water).

Contact precautions for MRSA, VRE, C. difficile, and multidrug-resistant gram-negative organisms; droplet for influenza and pertussis; airborne for TB, measles, and varicella.

Central line-associated bloodstream infection prevention: bundle including hand hygiene, maximal barrier precautions, chlorhexidine skin antisepsis, optimal site selection, and daily review of line necessity.

Catheter-associated UTI prevention: avoid unnecessary catheters, use closed drainage, and remove promptly; asymptomatic bacteriuria should not be treated except in pregnancy or before urologic procedures.

Antimicrobial stewardship core strategies: preauthorization, prospective audit with feedback, de-escalation, dose optimization, and duration limits.

Surgical site infection prevention: appropriate timing of prophylactic antibiotics within 60 minutes before incision, and discontinuation within 24 hours after surgery.

Outbreak investigation: case definition, line list, and analytic study (cohort or case-control) to identify exposures; hand hygiene and environmental cleaning are standard control measures.

Ventilator-associated pneumonia prevention: head-of-bed elevation, sedation interruption, oral care, and avoidance of unnecessary circuit changes.

Neutropenic fever: start empiric anti-pseudomonal beta-lactam promptly; add vancomycin for suspected line infection, mucositis, or hemodynamic instability; add antifungal coverage for persistent fever.

Post-transplant timeline: early (first month) bacterial and candidal infections; intermediate (1-6 months) opportunistic infections including CMV, Pneumocystis, and Aspergillus; late (>6 months) community-acquired infections and late opportunistic disease.

CMV prophylaxis (valganciclovir) or preemptive monitoring is standard after high-risk solid organ transplant; CMV disease presents with fever, cytopenia, and organ-specific involvement.

Pneumocystis prophylaxis is indicated for CD4 <200 in HIV, and for transplant recipients on immunosuppression; trimethoprim-sulfamethoxazole is first-line.

Asplenia: encapsulated organisms (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) are the major risk; vaccinate and provide prophylaxis per guidelines.

Solid organ transplant candidates: pre-transplant screening includes HIV, hepatitis B and C, CMV, EBV, syphilis, and TB (IGRA); live vaccines are contraindicated after transplant.

Hematopoietic stem cell transplant: pre-engraftment risk is bacterial and candidal; post-engraftment risk includes CMV, Pneumocystis, and Aspergillus; late risk is encapsulated bacteria and VZV.

Biologic immunosuppression (TNF inhibitors): screen for latent TB and hepatitis B before initiation; reactivation risk is high.

HIV diagnosis: fourth-generation antigen/antibody test is the screening standard; confirm with HIV-1/2 antibody differentiation assay, and use HIV RNA when results are indeterminate.

Initial ART: integrase inhibitor-based regimen (e.g., bictegravir/tenofovir alafenamide/emtricitabine or dolutegravir plus two NRTIs) is preferred for most patients.

PEP: start within 72 hours of exposure, use a 28-day three-drug regimen, and test the source and exposed person for HIV, hepatitis B and C, and other STIs.

PrEP: tenofovir/emtricitabine or tenofovir alafenamide/emtricitabine; monitor HIV status, renal function, and STIs every 3 months.

Syphilis: benzathine penicillin G is first-line for all stages; penicillin-allergic pregnant patients require desensitization and penicillin.

Gonorrhea: ceftriaxone 500 mg IM is first-line; test for cure in pharyngeal infection and in pregnancy, and screen for chlamydia coinfection.

Chlamydia: doxycycline 100 mg twice daily for 7 days is preferred for nonpregnant adults; azithromycin is an alternative in pregnancy.

HIV opportunistic infection prophylaxis: trimethoprim-sulfamethoxazole for Pneumocystis when CD4 <200; azithromycin for MAC when CD4 <50; toxoplasma prophylaxis when CD4 <100 and IgG positive.

Yellow fever vaccine is live and contraindicated in severe immunocompromise and pregnancy except when risk is high; a single dose provides long-term protection.

Typhoid vaccine: inactivated parenteral or live oral; live oral is contraindicated in immunocompromised patients.

Rabies post-exposure prophylaxis: wound washing plus vaccine series; rabies immunoglobulin is infiltrated at the wound for category III exposures and for immunocompromised patients.

Tetanus post-exposure: wound care plus tetanus toxoid booster; tetanus immunoglobulin for contaminated wounds with unknown or incomplete vaccination.

Hepatitis B post-exposure: vaccine series plus hepatitis B immunoglobulin for susceptible exposed persons; test source and exposed serology.

Malaria chemoprophylaxis: atovaquone-proguanil, doxycycline, or mefloquine by region; mefloquine is avoided in patients with psychiatric or seizure history.

Meningococcal vaccine: recommended for travelers to the meningitis belt and for asplenic or complement-deficient patients.

Vaccine contraindications: live vaccines (MMR, varicella, yellow fever, live oral typhoid) are avoided in pregnancy and severe immunocompromise; inactivated vaccines are preferred.

Blood cultures: obtain at least two sets from separate sites before antibiotics; each set includes aerobic and anaerobic bottles; do not draw through existing lines unless line infection is suspected.

Gram stain: gram-positive organisms retain crystal violet-iodine and appear purple; gram-negative organisms decolorize and counterstain pink.

Sputum culture is only useful if the specimen is adequate (many squamous cells, few polymorphonuclear cells); otherwise it reflects oral flora.

C. difficile testing: use a two-step algorithm (GDH or NAAT plus toxin enzyme immunoassay) for best positive predictive value; do not test formed stool.

Procalcitonin: rises in bacterial infection and falls with effective therapy; it can support antibiotic discontinuation decisions but is not a stand-alone diagnostic test.

Antimicrobial susceptibility: MIC is the lowest concentration inhibiting visible growth; interpret with CLSI or EUCAST breakpoints, and report ESBL and carbapenemase results.

Rapid diagnostics: multiplex PCR panels detect organisms and resistance genes quickly, but do not distinguish colonization from infection and should be interpreted with the clinical picture.

Urine culture: colony counts alone do not distinguish infection from asymptomatic bacteriuria; pyuria and symptoms are needed to support treatment.