Anatomy of an AST

LifeScale's ability to rapidly determine microbial growth makes the instrument an ideal platform for phenotipic antimicrobial susceptability testing. For Gram-negative rods this is usually in 4 hours, direct from positive blood culture. The images below outline the stages from well plate inoculation to the generation of a final AST report. 


Immediately following the inoculation of the plate the positive control wells are measured to verify the inoculm load. Data from the initial measurement can be seen, left side of the plotted data, in the first 5 minutes. After approximately 3 hours of on-instrument incubation the positive controls are re-measured and the growth delta from the initial inoculation concentration calculated. If the positive controls have undergone sufficient growth, the remainder of the inoculated wells are measured, right side of the plotted data.

Each of the 96 wells is measured and a microbe mass distribution obtained.

Raw LifeScale data, in the form of individual microbe mass, is obtained for each of the 96 wells on the plate. Each plotted point represents the mass of a single microbe.
The raw LifeScale mass and count data is reduced to a single microbe concentration value, #/ml, for each well. The plot on the right shows the calculated concentration for each well.    
Raw LifeScale mass data is converted to concentration, #/ml.
With a single value, concentration, determined for each well, a growth treshold can be calculated. The growth threshold is determined by the amount of growth, in the positive control wells, over the incubation period.    
A growth treshold is applied to the concentration data and a well-by-well "Grow/No Grow" determination made.
The growth threshold is applied to the data to generate a well-by-well "grow/no grow" value. Wells that show a 10x increase in concentration are considered to have "grown".    
A well-by-well growth determination is calculated for each of the wells on the plate.
The "grow/no grow" data is applied as a function of antibiotic, yielding an MIC value for each antimicrobial on the plate.    
MIC values are determined for each antimicrobial on the plate.


LifeScale is able to generate an MIC report from most Gram-neagtive rods in 4 hours. With the availability of an organism ID, not generated by LifeScale, SIR determination is made in accordance with  FDA/CLSI/EUCAST or local breakpoints. 


Final AST report is available once the organism ID is known.