CPT2: patient factors affecting anti-microbal selection Flashcards
What are the 3 main factors affecting anti-microbal selection
Three main factors:
- Known or likely causative organism
- local sensitivities & resistance patterns - Patient factors- allergies, renal or liver disease, immunosuppression etc
- Drug factors- contraindications or cautions
How do allergies affect choice?
History of allergy
- Penicillins
- Cephalosporins
- Sulphonamides (part of co-trimoxazole)
Patients with a history of atopic allergy (e.g. asthma, hay fever, eczema) are more likely to be allergic to penicillins. If allergic to 1 penicillin you are allergic to all.
If allergic to penicillin, may be allergic to cephalosporins.
Common hypersensitivty reaction in elderly patients e.g. skin reactions when using sulphonamides
How does renal function affect choice?
Many drugs are renally excreted and may also affect renal function themselves.
Renal Function
- Aminoglycosides e.g. gentamycin
- Tetracycline
- Nitrofurantoin
- Amoxicillin
- Vancomycin
Gentamycin and Vancomycin need to have TDM. These drugs both rely on renal function to calculate intial dose.
How does liver function affect selection?
Liver Failure
- Choramphenicol
- Co-amoxiclav
- Co-trimoxazole
- Tetracyclines
Choramphenicol is metabolised by liver enzymes therefore hepatic monitoring is required.
Co-amoxiclav = amoxicillian and clavulanic acid. Clavulanic acid component means hepatic monitoring required
Co-trimoxazole - the sulphonamide component is metbolised by hepatic enzymes so hepatic monitoring required
Can patient tolerate drugs by mouth?
What can affect this?
Ability to tolerate drugs by mouth
- Concomitant disease states e.g. Crohn’s, UC
- Vomiting & diarrhoea
- Dysphagia
How does the severity of illness affect selection?
Severity of illness
•Previous treatment failure with oral antibiotics.
How does being a female affect selection?
- Pregnancy
- Aminoglycosides e.g. gentamycine (toxic to feotus)
- Tetracycilnes (prevent skeletal development therefore not used in children)
- Quinolones e.g. ciprofloxacin (Contraindicated in children. Special cases may use in child)
- Trimethoprim (terotogenic in trimester 1)
- Nitrofurantion (terotogenic in trimester 3)
- Breast feeding:
- Rule of thumb - safe in pregn = safe in breast feeding
- Avoid ciprofloxacin and tetracyclines
- Oral contraception
How does dehydration affect selection?
Dehydration/inadequate fluid intake
- Can lead to crystalluria - crystals in urine
- Specifically with ciprofloxacin Sulphonamides
- Counsel on importance of fluid intake
How does glucose-6-phosphate dehydrogenase deficiency affect selection?
Glucose 6-phosphate dehydrogenase (G6PD) deficiency
- Haemolytic anaemia can develop in those defieceny of enzyme. Without this enzyme some drugs are impossoble to metabolise
- caution with sulphonamides, ciprofloxacin and nitrofurantoin
- Susceptibility to the haemolytic risks from drugs varies
- Risk & severity of haemolysis is almost dose-related
- Need to avoid certain foods - check chap 9 anaemia bnf
How does cardiac failure affect selection
Parental borad spectrum penicillins have high Na+ content
How does diabetes affect selection?
- Impairs healing process
- Diluatn glucose or dextrose - affect sugar levels
How does epilepsy affect selection?
Some drugs lower epileptic threshold
DRUG FACTORS
What are drug factors which affect selection?
- Toxicity/unwanted effects
- Formulation
- GI absorption
- Tissue levels
- Half life
- Drug interactions
- Synergistic/additive effects
- Cost
What are unwanted side effects of ABs
- Nausea
- Particularly with erythromycin, doxocycline and nitrofurantion
- Diarrohea
- Oral/anogenital thrush/ pruritis (rash)
- especially with sulphonamides and penicillins
- Caused by broad spec see below
- Broad spectrum AB can cause fungal infections. AB associated collitis, other problems associated with suprainfections e.g. vagitis and pruritis
- This is due to AB killing goof and bad bacteria leading to overgrowht of other bacteria
Treatment of unwanted effects:
- Vaginitis
- Candidosis
- AB associated colitis
- Vaginitis - metronidazole
- Candidosis - topical or oral anti-fungal
- Antibiotic associated colitis likely to be due to a C.difficile infection - oral metronidazole, oral vancomycin, Fidaxomicin (Dificlir™) [Latter for management of recurrent C.difficile infection]
- Vancomycin not absorbed from GI tract therefore useful
- Fidaxomicin licenced for reccurent C.diff infections
What are specific side effects of:
- Aminoglycosides
- Vancomycin
- Doxycycline
- Trimethoprim
- Sulphonamides
- Penicillins
- Cephalsporins
Penicillins Allergy
- (5-10%)urticarial rash(5%)
- Joint pain,malaise,local oedema,anaphylaxis
Cephalosporins
- up to 6.5% patients cross-allergic with penicillins
Trimethoprim
- bone marrow depression in folate deficient patients, potentially teratogenic
Aminoglycosides & vancomycin
- ototoxicity, nephrotoxicity
Sulphonamides
- Stevens-Johnson syndrome & hypersensitivity, blood dyscrasias due to bone marrow depression and agranulocytosis
Doxycycline
- Photosensitivity
What should be adviced to patients taking doxycycline
avoid sunbathing or sun beds
What routes are available for administraion?
- Oral (most common)
- Parental
- Rectal
- Nebulised
- Infections associated with CF
- Topical
Describe GI effect of some ABs
- Ampicilin/amoxicillin- 30-70% absorbed
- gentamicin, ceftazidime, vancomycin - poor absorption - give by injection
How does half-life affect selectivity?
Half life
t1/2 – will affect the dosing frequency so indirectly may affect compliance
How does tissue level affect choice?
Want high level of drug in tissue where infection is
- CSF - chloramphenicol, cefotaxime (meningitis)
- Bones - flucloxacillin, fusidic acid
- Prostate - Trimethoprim, ciprofloxacin
- Urinary exretion - all excreted bar chloramphenicol
Drug interactions
- Food - Decreases plasma levels of many antibacterials
- Exception – Nitrofurantoin
- Combined OC’s (decreased effect) - with rifampicin
- Progesterone-only OC’s (decreased effect) – rifampicin
- Alcohol - with Metronidazole leads to disulfuram reaction - nausea & vomiting
- Antacids/iron/zinc - decrease effect of tetracyclines, doxycycline & ciprofloxacin
- Warfarin
- Antiepileptics
- Rifampicin decreases efficitivty
- Theophylline
Rifampicin METABOLISES ALL DRUGS
MACROCYCLIC AB
