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 CF S RFRFR TS TR FSFSAS AR ACAF Clinical Response ASTAST

Percent Clinical Success S I R NitFurCefotetCefotax1Cefotax2CipFlox1CipFlox2 Poupard JA, Walsh RA, Kleger B. Antimicrobial Susceptibility Testing. New York: Plenum, 1994;

Predictive Values of AST NitFurCefTetCefTax1CefTax2CipFlx1CipFlx2 PVsPVr

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 Report Specimen: Urine Isolate: E. coli Antibiotic Penicillin G Sulfadimidine Nitrofurantoin Coamoxiclav Ciprofloxacin P of Success

Percent Clinical Success S I R NitFurCefotetanCefotax1Cefotax2CipFlox1CipFlox2 Poupard JA, Walsh LR, Kleger B. Antimicrobial Susceptibility Testing. New York: Plenum Press, 1994;

Percent Clinical Success S I R NitFurCefotetanCefotax1Cefotax2CipFlox1CipFlox2 Poupard JA, Walsh LR, Kleger B. Antimicrobial Susceptibility Testing. New York: Plenum Press, 1994;

Antimicrobial Susceptibility Tests vs. Clinical Response Empyema due to S.  -hemolyticus “susceptible” to penicillin G No response to high doses of penicillin G Aspirated pus contained no penicillin, and also inactivated added penicillin Rapid recovery when treatment changed to doxycycline! [Barnes P, Waterworth PM. BMJ 1977 Apr 16; 1:991-3.]