We likewise use 5 day protocol on blood cultures, and unfortunately grow
quite a few CNS's. We get blood cultures from four hospitals, as we're a
regional facility. Skin contamination is alive and well. We do
sensitivities on all of them and let the docs sort it out. If only one
of a set of 2 or 3 we append a long coded comment to the effect that one
isolated CNS may represent skin contamination.
We hold negative broths and anaerobic plates on anaerobe cultures five
days before discarding. We only subculture visibly positive cooked
meats (due to extremely high volume. I had 37 new specimens on that
bench yesterday. Multiply this X 5-6 days and it gets really out of
hand. Thankfully only about 1/4 of them grew anything, as we get many
surgical specimens. The ones that grow, though, are NASTY). Aerobic
plates on these specimens are discarded at 48 hours if no growth. Our
Ph.D. is in the process of evaluating our whole anaerobe procedure and
even debating about setting up a broth backup in the first place,
although I tend to doubt if he'll drop it. At my former employer we
held all "no growths" three days before discarding. There we stained
visibly clear broths before signing out.
I have never worked anywhere where we held the bacterial specimens for
"weeks" before signing out. How does ASM know after that amount of time
that it hasn't become contaminated from all that handling? The CNS I've
seen tends to grow at max by 48 hours, and is usually there in 24 on
plates. Blood cultures tend to take longer if they are there in low
numbers. I've seen them turn positive on the third or fourth day max.
Very rarely have I seen true pathogens come up after the third day,
although it does happen.
Judy Dilworth, M.T. (ASCP)
>> I read the 'Manual of Clinical Microbiology', published by ASM, a while ago,
> but I seem to remember that coagulase negative Staphs can take even weeks to
> grow particularly from prosthetic heart valves. We sign out our negative
> cultures in 3 days but hold the broth cultures for 7 days, screening them daily
> for growth before we discard them. We hold blood cultures for 5 days before
> reporting them as negative. For more information, I suggest you read the
> chapter on Staph in the 'Manual of Clinical Microbiology'.