Post on 19-Jan-2016
Antimicrobial Susceptibility Tests
• Measure either MIC or zone diameter
• Report result as S, I or R
• Clinical utility depends on predictive value (PV)
• PV rarely reported in clinical trials
• Can we make it a reporting standard?
• If it is poor, can we improve it?
Predictive Value of AST
• PV = T / (T+F)• PVs = TS / (TS+FS)
= TS / AS• PVr = TR / (TR+FR)
TR / AR
C F
S
R FR
TS
TR
FS AS
AR
AC AF
Clinical Response
AST
0
50
100
Per
cen
t C
lin
ical
Su
cces
s
S 90 94 93 92 89 91
I 73 96 78 83 90 79
R 50 77 65 63 86 62
NitFur Cefotet Cefotax1 Cefotax2 CipFlox1 CipFlox2
Poupard JA, Walsh RA, Kleger B. Antimicrobial SusceptibilityTesting. New York: Plenum, 1994; 15-25.
Predictive Values of AST
0102030405060708090
100
NitFur CefTet CefTax1 CefTax2 CipFlx1 CipFlx2
PVs PVr
Antimicrobial Susceptibility Testing
• Isolated pathogen- is it likely to be the cause?
• Site of infection- expected antimicrobial concentration?
• MIC or zone diameter• Immune status of the patient• Dose, frequency and route of the drug
[Evans ER et al. NEJM 1998 Jan 22; 338:232-8.].]
Clinically Useful Report
P = a MIC + b g/mL + c IS
g Dose, Route, Frequency, PK
If P = probability of cure with an antibiotic, then
Where
and IS = immune status
Clinically Useful ReportSpecimen: Urine
Isolate: E. coli
Antibiotic• Penicillin G• Sulfadimidine• Nitrofurantoin• Coamoxiclav• Ciprofloxacin
P of Success
0.40
0.80
0.90
0.85
0.90