Approach towards Dx - infectious disease
Age? Sex? Race? Socioeconomic status?
Symptoms and signs?
What is the expected pathogen for a given infection?
Allergies?
Medications? Immunosuppressed drugs?
Occupation and living conditions? Climate, season, time of day?
Immnization history?
What are the susceptibility trends - institutional, local, regional, national, global?
Geographic predisposition, immigration from? travel?
Environment. H/o of exposure to sick contacts, animals, or potentially contaminated environments?
Behavior. High risk behavioral factors? Sexual h/o, and use of contraception method?
History of prior illness? Noscomial infection? HCAP? MSSA/MRSA, VRE, ACB, ESBL, KPC?
IV devices, surgical incision, breach of skin barrier; breach of mucosal surfaces (ETT, bladder catheters), FB (implanted devices).
ABx alteration of natural flora. C.difficle
Comorbidities (chronic diseases)
Emotional state, psychiatric issues?
Pregnancy & lactation?
Nutrition?
Social history?
Gram stain. Rapid diagnostic testing, such as PCR and antigen detection. Culture and sensitivity results - narrowest spectrum antimicrobial?
Urgent therapy is indicated in febrile patients who are neutropenic or asplenic; however, in other immunosuppressed patients, fever alone seldom warrants urgent therapy. Sepsis, meningitis, and rapidly progressive anaerobic or necrotizing infections should also be treated promptly with antimicrobials.
In the acute clinical scenarios, empiric therapy is usually begun immediately after appropriate cultures have been obtained. However, if the patient's condition is stable, delaying the empiric use of antimicrobials allows for specific therapy based on the results of initial tests and avoids the use of unnecessary drugs.
Phamacokinetics:
Biovailability?
Can parenteral Tx be switched to PO Tx?
Interactions?
Renal/hepatic insufficiency?
Assessment of antimicrobial therapy: When therapy is reviewed as possible treatment failure, consider the following questions:
Is the isolated organism the etiologic agent?
Is adequate antimicrobial therapy being provided?
Is the concentration of antimicrobial agent adequate at the site of infection?
Altered pharmacokinetics secondary to drug interactions.
Have resistant pathogens emerged?
Is a persistent fever due to underlying disease, abscess formation, an iatrogenic complication, a drug reaction, or another process?
Impaired immune host defenses
Duration of therapy:
Treatment of acute uncomplicated infection should be continued until the patient has been afebrile and clinically well, usually for a minimum of 72 hours.
Infections at certain sites (e.g., endocarditis, septic arthritis, osteomyelitis) require prolonged therapy.
Pregnancy and postpartum patient:
PCN and cephalosporins are relatively safe
Tetracyclines and FQ are contraindicated.
Avoid sulfonamides and aminoglycoside
Most adequately dosed ABx appear in breast milk and should be used with caution in patients who are breast-feeding.
Diagnostic methods:
Methods used to identify pathogen: Microscopic examination, growth or biochemical characteristics in culture, immunologic techniques to identify antigens or antibodies. DNA probes.
Stains used to identify pathogens: Gram stain, acid-fast stain, Ziehl-Nielsen, Kinyoun, Wright's, KOH, India ink, Gomori's methanamine silver, Tzanck.
Acid-fast stain: Mycobacteria have a high lipid and wax content in the cell wall that resist staining, but, once stained, it is not decolorized, even by acid alcohol (call acid fast). The acid-fast organisms are red against a blue-green background.
Cultures:
Throat - 90% sensitive for strep pharyngitis
Lower resp - fewer than 10 epithelial cells and more than 25 PMNs/LPF needed for adequate sputum specimen.
Urine - needed "clean-catch" midstream urine specimen. If a catheter is in place, disinfect the tubing with a sterile needle and syringe. Do not collect it from the bag.
Blood cultures - avoid femoral veins and areas of indwelling catheters, for which there is a higher rate of contamination. In general, draw two sets to avoid obtaining one positive culture with a potential contaminant.
Body fluids - collect in a sterile fashion.
CSF - collect serum simultaneously for glucose determination.
Viral - use special transport media if the specimen is from throat or skin; buffy coat for HSV and CMV.
Direct immunofluorescence: Antigens or antibodies are directly labeled and detected by fluorescent microscopy.
PCR: Uses enzyme DNA polymerase to increase the number of copies (amplify) of DNA or RNA in a sample. PCR is very sensitive because only a few copies of genetic material (and not whole organisms) needed to be present. It is useful for organisms that are difficult to culture (including HIV).
Serologic testing: Used to diagnose infection when the pathogen cannot be cultured. Measure acute and convalescent sera to detect a fourfold increase in titer (synonymous with recent infection). Diagnosis can only be made retrospectively.
Animal contact: rabies, Q-fever, bartonellosis, E. coli O157, cryptococcosis.
Blood products: viral hepatitis, malaria, prion disease
Insect vector + season + geographical site of exposure: RMSF, rickettsial disease, tularemia, Lyme disease, Babesiosis, malaria, trypanosomiasis and numerous arboviral infections.
Ingestion of contaminated liquids and foods: enteric infection - Salmonella, Listeria, Campylobacter, amebas, cryptosporidia, or helminths
ROS
Physical Exam: Heart murmur, fundus - retinal lesions (CMV, candidiasis). Rashes - childhood exanthems (measles, rubella, varicella), erythema migrans (Lyme), erythema gangrenosum (Pseudomonas aeruginosa), and eschars (rickettsial disease). Early scarlatiniform and later petechial rashes of meningococcal infection. Embolic lesions: disseminated fungal infection (immunosuppressed). Fever, q48-72h - malaria.