Friday, July 10, 2020

What is a "false positive" culture?

The Incidence and Incubation Period of False Positive Cultures in Shoulder Surgery

These authors studied 95 patients having either an open and arthroscopic procedure group (excluding those with abnormal inflammatory labs, history of previous shoulder surgery, or corticosteroid injection within six months of surgery). There were 47 patients in the open group and 48 in the arthroscopic group. In the open group, surgeries included 22 replacement arthroplasties, 22 rotator cuff repairs, and 3 instability repairs. In the arthroscopy group, surgeries included 24 acromioplasties, 17 rotator cuff repairs, 4 distal  clavicle resections, and 3 instability repairs.

Three cultures were obtained for each patient: superficial tissue culture, tissue culture, and  “sterile” control swab. Cultures were held for 28 days and checked on regular intervals.

In the open group, positive culture results were reported for eight of the forty-seven patients (17.02%) with one patient yielding positive superficial and deep culture specimens, each growing a different organism. Nine of the 141 cultures (6.3%) obtained were positive for bacterial growth. Five positive cultures were from superficial tissue, two positive cultures were isolated from deep tissue, and two positive cultures were isolated from the control “sterile” swab  group. All positive bacterial cultures were reported within seven days.  C. acnes was isolated in three of the forty-seven patients (6.4%), twice from a superficial tissue specimen and once from a deep tissue specimen. C. acnes was the most common isolated organism (33% of positive cultures) in the open group followed by Coagulase-negative Staphylococcus aureus (CoNS, 22% of positive cultures). All three patients yielding positive C.  acnes growth were men.

In the arthroscopic group, positive culture results were reported five of forty-eight patients (10.4%). Eight of the 144 cultures (5.5%) were positive for bacterial growth and one of the 144 cultures (0.7%) was positive for “mold”. Of the nine positive cultures, three were  obtained from superficial tissue, three were obtained from deep tissue, and three were obtained from the “control” swab. Two patients had more than one positive culture with one of the patients growing Methicillin Resistant Staphylococcus aureus (MRSA) from the superficial, deep, and “sterile” control cultures. All positive bacterial culture results were reported within seven 158 days. Only one specimen was isolated beyond one week, which was positive with “mold” at twenty-six days.  No specimens from the arthroscopic group were positive for C. acnes. The most common isolated organism was MRSA (50% of positive cultures) followed by CoNS (25% of positive cultures). 

They reported a "false-positive rate in open shoulder surgery of 17.02% and 10.4% in arthroscopic shoulder surgery.

The incidence of positive C. acnes cultures was 6.4% in the open group while C. acnes was not isolated in the arthroscopic group. All positive bacterial cultures were reported within seven days  of collection. 

Holding cultures longer than 14 days did not lead to an increased rate of false positive culture results.

In their Discussion, the authors pointed to " disturbing findings when reviewing the results of our study. One concern, was the high rate of lab errors. We had to exclude five patients due to blatant errors, including lost samples and removing cultures prior to 28 days. Additionally, five of the eighteen positive cultures were from the “sterile” control swabs, which suggest a high rate of contamination during sampling, transport, or laboratory handling."

Comment: The authors state in their discussion, "We are not able to prove the positive cultures are truly false positives." This begs the question, "what does it mean to state that a culture is "falsely positive"?" It seems more rational to state that no positive culture is "falsely" positive: if bacteria grew, the culture was positive; end of story. Positive cultures from "control" swab specimens are not "falsely positive": somewhere along the line the swab or the medium in which it was placed or the plates on which it was cultured acquired bacteria - truly.  Surgeons should periodically submit control swabs as quality assurance of their culturing system; see this link. If there is a substantially positive control swab it's time to check each step of the process to see where the bacteria are coming from (fingers, gloves, swabs, air) and when. Positive control swabs confound the interpretation of culture swabs from patients - patient cultures cannot be properly evaluated without knowing the results of local control swabs.

Similarly, positive cultures from specimens obtained from asymptomatic patients are not "false positives", although they may be unexpected. The bacteria cultured must have come from somewhere. If concurrent control swabs are negative, it is likely that the bacteria came from the patient - truly. That doesn't mean that the bacteria were doing harm, just they were there.

Finally, assessing the load or degree of positivity of bacteria in a cultured specimen can be helpful in interpreting the result. See this link and see this link.

To see our new series of youtube videos on important shoulder surgeries and how they are done, click here.

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