A comparison of blood agar supplemented with NAD with plain blood agar andchocolated blood agar in the isolation of Streptococcus pneumoniae and Haemophilus influenzae from sputum

Streptococcus pneumoniae grows well and generally exhibits typical morpholo gy on Columbia blood agar, whereas Haemophilus influenzae requires a more c omplex medium to meet its growth requirements - usually chocolated blood ag ar - on which S. pneumoniae is less easily recognisable. Therefore, a singl e medium that produces typical morphology of S. pneumoniae and facilitates the growth of H. influenzae would have considerable potential advantages. I t has been claimed that blood agar supplemented with nicotinamide adenine d inucleotide (NAD) is such a medium. However, despite its routine use in sev eral large diagnostic laboratories its performance has never been properly evaluated. In the present study, 1724 sputum samples were examined in four laboratories. The isolation rates of H. influenzae and S, pneumoniae on NAD -supplemented blood agar (SBA) were compared with those on a two-plate comb ination of plain blood (BA) and chocolated blood agar (CBA), The two-plate combination performed significantly better for both organisms; isolation ra tes for H. influenzae were increased from 8.16% on SBA to 11.07% on BA plus CBA and for S. pneumoniae from 4.18% to 4.68%. Isolation rates were also c ompared after incubation for 24 and 48 h. With the two-plate combination, i solation rates for H. influenzae and S, pneumoniae were increased by 0.98% and 0.16%, respectively, and for SEA by 0.57% and 0.32% after 48 h. However , despite this increase, SEA still performed less well than the two-plate c ombination.