for 18 h. Zones of inhibition were measured using micrometers, and
the norfloxacin concentrations in the test samples were calculated
from the standard curve. The method was validated in terms of
linearity, sensitivity, recovery, intra-day and inter-day precision.
Semi-logarithmic plots of the inhibition zone diameter, versus
standard norfloxacin concentrations in serum, were linear between
0.05 and 20 μg ml-1, with a typical correlation coefficient of 0.990 (for
the standard curve). The limit of quantification of the norfloxacin
assay was 0.05 µg ml-1. The percentage recoveries were determined
by comparing the inhibition zones of blank samples spiked with
different amounts of drug and treated as any sample, with the
inhibition zones of the same standards prepared in phosphate buffer
(n=6). The mean percentage recovery of norfloxacin from serum was
96.17 ± 6.79%. Intra-assay variations were determined by measuring
six replicates (n=6) of three standard samples used for calibration
curves [low (0.1 µg ml-1), medium (1 µg ml-1), and high (10 µg ml-1)]
on the same day. The intra-assay variation coefficient was 3-5% for
serum. An inter-assay precision was determined by assaying the three
standard samples on three separate days. The inter-assay variation
coefficient was 5-8% for serum. The intra- and inter-day precision
and accuracy of the assay were determined by percent coefficient of
variation (CV). The coefficient of variation was calculated as follows:
CV (%) = (standard deviation/mean) ×100%