Method of respiratory specimen collection significantly influences the identified etiology of VAP. This observational study in seven Polish ICUs characterized the etiology of VAP when diagnosed by a quantitative culture from BAL, contamination-protected brush, or distal protected aspirate (criteria collectively identified as PNEU-1 by the European CDC) versus when diagnosed by other methods (e.g. non-protected samples like endotracheal aspirate, or non-quantitative culture). The authors identified 205 VAP cases, of which 39% had a microbiologic diagnosis. The etiology of VAP varied significantly based on whether PNEU-1 or alternative culture methods were used: patients diagnosed using PNEU-1 methods had infections mostly caused by S. aureus and K. pnuemoniae (21.3% each), whereas cases diagnosed by alternative methods were more likely to be due to A. baumannii (30.2%). PNEU-1 methods also identified more P. aeruginosa (10% vs 2% for other methods).
Why should we care? Well, I think this study is important because many A. baumannii are highly resistant to beta-lactams, including carbapenems (53-71% of isolates were resistant to carbapenems in this study; in general, carbapenem resistance in A. baumannii seems to be more common than in other gram-negative bacilli). Thus, overdiagnosis of A. baumannii could be a driver of non-preferred antibiotic use (e.g. colistin, aminoglycosides) and excess morbidity and mortality for VAP diagnosed by suboptimal methods. Although the paper doesn’t provide a PNEU-1 vs non-PNEU-1 breakdown of antibiotic use for the VAP cases, I think it’s worth noting that VAP cases diagnosed by PNEU-1 criteria received shorter durations of antibiotics (7 vs 9 days) and had shorter hospitalizations (49 vs 52 days), which could reflect better outcomes related to more beta-lactam use. 29976151
There’s a new, modestly more sensitive urinary pneumococcal antigen in town. This observational study compared the new SofiaFIA urine antigen to the BinaxNOW antigen for the diagnosis of pneumococcus in patients with a clinical diagnosis of pneumonia. Pneumococcal pneumonia was defined as proven (isolation of the organism from sterile fluid) or probable (isolation of the organism from a respiratory sample). I thought this was an interesting definition, as I’d always consider a sputum culture growing pneumococcus significant in a patient with a clinical diagnosis of pneumonia. Anyway, the authors performed both urine antigens on each patient. Out of 219 patients, 14% had proven or probable pneumococcal pneumonia, 22% had another etiologic diagnosis, and the remainder had no etiologic diagnosis. The SofiaFIA antigen was significantly more sensitive than and as specific as the BinaxNOW antigen; among the 64% of patients with no identified organism, SofiaFIA was positive in 24% of cases versus BinaxNOW’s 18%.
Put another way, if your hospital is using the BinaxNOW antigen and switches to the SofiaFIA antigen, and the etiology of pneumonia at your hospital is similar to this hospital in Spain, these data suggest you might reclassify ~13% of your currently undiagnosed pneumonias as pneumococcal pneumonia. 29651615
How do the sensitivities and specificities of the various treponemal tests for syphilis compare? This study compared seven treponemal assays to a reference standard consisting of clinical diagnosis plus the consensus of serologic testing. The assays included two chemiluminescence immunoassays (ADVIA CIA and LIAISON CIA), a microbead immunoassay (MBIA), an enzyme immunoassay (EIA), a line immunoassay (INNO-LIA), and the traditional assays FTA-ABS and TP-PA. Sera from 959 individuals were collected; patients were categorized as having a clinical diagnosis of syphilis (primary, secondary, early latent, or late latent), prior treated syphilis, or no syphilis. ‘No syphilis’ was defined as the absence of current clinical suspicion for syphilis, no prior history of the disease, and a negative result on the majority of the seven assays. The FTA-ABS proved less sensitive for primary syphilis than the other assays (78% vs 95-96%); all assays were 100% sensitive for secondary syphilis, 95-100% sensitive for early latent disease, and 87-99% sensitive for late latent disease. The most specific test is still the TP-PA, which had 100% specificity in this study. 29986091
Fosfomycin susceptibility testing still sucks. Fosfomycin is one of the ID doc’s secret weapons, reliable for UTI even in patients with highly resistant organisms because nobody else uses it. And why don’t they? Probably because they don’t think about it, because it doesn’t show up on urine culture susceptibilities. Why’s that? It’s complicated, but fosfomycin susceptibility testing is tricky and needs to be done by agar dilution: here’s a nice run-down of the technical issues (thanks to Dr. Rich Davis for the link). This study out of the Netherlands collected 976 isolates of ESBL E. coli and K. pneumoniae and tested them for fosfomycin susceptibility using agar dilution as well as various other methods (i.e. disc diffusion, E test, MIC test strep, Vitek2, and Phoenix) to see how reliable said methods were. Note this study uses EUCAST rather than CLSI breakpoints as this was a European study.
The good news is that fosfomycin susceptibility rates were high: 96% for ESBL E. coli and 88% for EBSL K. pneumoniae. The bad news is that none of the other susceptibility testing methods correlated reliably with agar dilution; error rates ranged from 12% (Phoenix) to 23% (E test). With the US approval for IV fosfomycin, hopefully someone will come up with a better susceptibility test soon. 29982660
MALDI-TOF can be used for rapid detection of antibiotic resistance! The simplest way I can explain Matrix Assisted Laser Desorption/Ionization Time-of-Flight mass spectrometry (MALDI-TOF) is that it identifies microorganisms by protein phenotype. Every bacteria and fungus has its own ‘fingerprint’ consisting of what proteins it produces and in what ratios it produces them, and MALDI-TOF reads these fingerprints and then compares them to the prints in its database of known pathogens. So, knowing that, can we use MALDI-TOF to identify phenotypes of antibiotic resistance? Turns out yes, we can.
First, the authors incubated 24 strains each of K. pneumoniae and P. aeruginosa in microdroplets (2 to 10 microliters) on the MALDI-TOF target with and without meropenem. Second, the broth was removed with an absorbtive material, leaving only the cells, and the MALDI-TOF assay was performed. The 6ul volume gave the best results, and after allowing 4-5hr of incubation the test was 100% sensitive and specific for meropenem resistance in both organisms. Between this approach and the new multiplex PCR assays detecting antimicrobial resistance genes directly from positive blood cultures, I think we’re entering a new era where the turnaround time from empiric to definitive antimicrobial therapy could regularly be less than 24 hours. That’s great news for antimicrobial stewardship – so long as someone is actually looking at and acting on the results of these new tests. 29079147
Time-lapse microscopy is a novel method to test antimicrobial regimens against MDR gram-negative bacteria. The authors took four strains of K. pnuemoniae and two strains of E. coli and grew them in media in the presence of colistin, meropenem, rifampin, and tigecycline alone and in various combinations. They used automated time-lapse microscopy over 24 hours to determine the background corrected absorption (If I understand correctly, this is similar to looking at change in optical density to quantify bacterial growth in broth culture). They found that BCA values <8 consistently correlated with the presence of <10^6 cfu/ml bacteria at 24hr. More importantly, they found that the effects of antibiotic combinations (i.e. synergy, indifference, or antagonism) as assessed by automated microscopy correlated well with the results of traditional time-kill studies, with agreement between the two methods occurring 33/36 cases. So, automated microscopy may be valuable as a time and cost-saving approach to evaluating the synergistic potential of antibiotic combinations for MDR bacteria. 29108951