Hi Yinkuse , I guess it has to do with the ratio of the number of resistance to antibiotics display by an isolate to the total number of antibiotics tested . In your case I guess it will mean for each of the 100 isolate, how many antibiotics each of the isolate is resistant to divided by 6. A completely resistant isolate would then have a MAR index of 1.0, just cross check

Hi Umar, thanks for the response, I saw another formula which multiply the total number of the isolates (100) by 6 as part of the step you described. So it kinda confusing on what is the right thing to do.

MAR index is calculated as the ratio of number of antibiotics to which organism is resistant to total number of antibiotics to which the organism is exposed. Bacteria having MAR index ≥ 0.2 originate from a high risk source of contamination where several antibiotics are used

a/b

Where (a) is the aggregate antibiotic resistance score of all isolates from the sample, (b) is the total number of antibiotics used

Just sharing the answer I found

https://scialert.net/fulltext/?doi=ajas.2015.233.240

Article Multiple antibiotic indexing of Escherichia coli to identify...

Multiple antibiotic resistance (MAR) index can be determined for by using the formula MAR = a/b, where a represents the number of antibiotics to which the test isolate depicted resistance and b represents the total number of antibiotics to which the test isolate has been evaluated for susceptibility

I think that MAR index was proposed by Krumperman in 1983 ( Appl. Environ. Microbiol. 46:165–170). MAR index of single isolate is defines as a/b, where a is a number of antibiotics which the isolate is resistant, and b is a number of antibiotcs tested. The MAR index of sample or sampling point (from which several isolates were taken) is defined as a/(bxc), where a is a aggregate antibiotic resistance score of all isolates from the sample or sampling point, b is the number of antibiotics tested,and c is the number of all isolates from the sample or sampling point.

However, in case of "clinical" isolates, like Enterobacteriaceae, including E. coli, St. aureus, Ps. aeruginosa and Acinetobacter spp. you can use terms MDR (multidrug-resistant), XDR (extensively drug-resistant) and PDR (pandrug-resistant) according to recomendations of EUCAST, published by Magiorakos et al in 2012 (Clinical Microbiology and Infection 18,3,268-281). MDR was defined as acquired non-susceptibility to at least one agent in three or more antimicrobial categories,

How is MAR calculated?

Multiple Antibiotic Resistance indexes (MAR) are found out by the formula:

📷MAR index of isolate = No. of antibiotics to which an isolate is resistant/

Total no. of antibiotics to which the isolate was exposed

The limits are basically to determine if the bacteria arise from any high risk source where antibiotic use rather misuse is predominant. While MAR index of 0.2 or higher indicates high risk sources of contamination, MAR index of 0.4 or higher is associated with human faecal source of contamination

MAR index when applied to a single isolate, is defined as a/b, where a represents the number of antibiotics to which the isolate was resistant, and b represents the number of antibiotics to which the isolate was exposed. For example, if the isolate were exposed to 10 antibiotics and were resistant to 5, the index for the isolate would be 5/10, or 0.50. If indexing is applied to a sample from which several isolates were taken, the index of the sample would be a/(b * c), where a is the aggregate antibiotic resistance score of all isolates from the sample, b is the number of antibiotics, and c is the number of isolates from the sample. For example, if the aggregate antibiotic resistance score of 20 isolates taken from a sample were 160, the MAR index of the sample would be 160/(10 x 20), or 0.8. (According to Krumperman, 1983). Values of MAR higher than 0.25, pose high risk source for contamination.