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