Jim Kling

September 18, 2013

DENVER — Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry shortens the time it takes to identify the agent responsible for a bloodstream infection and can have a significant effect on patient management, new research shows.

The new mass spectrometry technology has had a dramatic effect on the identification of clinical isolates because of its high-throughput capacity and rapid action. Still, few studies have demonstrated the impact of the technology on clinical practice.

"At the time our study was done, not much had been done to evaluate whether the reduced time to identification of causative agents affects clinical outcomes or antimicrobial prescribing," said Katherine Bond, MD, infectious diseases and microbiology registrar at St. Vincent's Hospital in Melbourne, Australia.

Dr. Bond presented the research here at the 53rd Interscience Conference on Antimicrobial Agents and Chemotherapy.

We went from 6% having an identification with Vitek 2 to 34% with MALDI-TOF. That makes a difference on the ward.

Her team conducted a retrospective review of positive blood cultures analyzed with the Vitek 2 microbial identification system (n = 66) or MALDI-TOF mass spectrometry (n = 53).

Investigators considered antimicrobial therapy appropriate if the organism was susceptible to the antibiotic prescribed and the drug was a first-line therapy for it, or if the drug was theoretically susceptible but was not a first-line choice.

The primary outcomes were time to correctly identify the pathogenic organism and achievement of appropriate antimicrobial therapy at 24 hours.

MALDI-TOF analyses were conducted with a Microflex LT MALDI-TOF MS (Bruker Daltonics) with BioTyper version 2.0 software.

MALDI-TOF spectral scores of 2.0 were considered correct to the species level, and results were released to the treating clinician. When the spectral score iwas 1.7 to 2.0, researchers consulted with a microbiologist, and some isolates were released to the clinician at the genus level.

MALDI-TOF identified 34.0% of organisms with a score above 2.0, and 54.7% with a score above 1.7.

Table. Antibiotic Selection With Vitek 2 and MALDI-TOF

Identification System n Inappropriate, % Appropriate, %
Vitek 2 66 15.2 84.8
MALDI-TOF score      
>2.0 18 0 100.0
1.7–2.0 11 9.1 90.9
<1.7 24 29.2 70.8


In the study, MALDI-TOF reduced identification time from 27 hours to 17 hours, although a starting point of 27 hours is probably lower than most facilities, according to Dr. Bond. "The standard might be around 40 hours, so our absolute decrease was not as great as other laboratories might gain. However, in the first 10 hours, we went from 6% having an identification with Vitek 2 to 34% with MALDI-TOF. That makes a difference on the ward," said Dr. Bond.

The study didn't show a significant improvement in appropriate antimicrobial therapy, but that's likely because the hospital already had a high rate — around 85% — even with the standard care. Other studies have shown that hospitals often begin with rates closer to 65% to 75%, and then improve with MALDI-TOF to 85% to 90%, "which is where we started," Dr. Bond pointed out.

She added that institutions considering adopting MALDI-TOF should look at their baseline appropriate antibiotic rate and local prescribing and resistance patterns. "Then you can get an idea of how much impact MALDI-TOF is going to have in your lab."

In a second study, led by Tranprit Saluja from Sandwell and West Birmingham National Health Service Trust in the United Kingdom, researchers conducted a retrospective analysis of the identification of blood culture isolates using the Vitek MS (bioMérieux) MALDI-TOF system, which was approved for clinical use by the US Food and Drug Administration in August.

The study involved 216 episodes of bacteremia from 144 patients. MALDI-TOF of blood culture broths correctly identified 177 of 216 episodes (81.9%) of monomicrobial bacteremia at the genus level. These results matched the organism recovered on subculture and were available 24 hours sooner.

In 19 polymicrobial blood cultures, MALDI-TOF correctly identified at least 1 of the organisms. For 35 blood culture bottles, no results were obtained because no database match was found or the spectra were insufficient for analysis; 4 bottles were incorrectly identified.

The researchers concluded that on day 1, Gram staining and MALDI-TOF direct identification would have prompted physicians to change therapy for 19 patients (13%).

These 2 studies attracted interest from others considering purchasing a MALDI-TOF mass spectrometer, including Sue Whitehead, a technical specialist in microbiology at the Interior Health Authority in Kamloops, British Columbia, Canada.

"We're hoping for a faster turnaround time and more targeted and earlier prescribing of antimicrobial therapy," Whitehead told Medscape Medical News. "We currently use the Vitek 2, which is a good system, but it takes 2 days or so. In Dr. Bond's study, it didn't make a big difference in clinical impact because they were already using a rapid technique. I think it'll be a bigger bang for the buck for us than it was for them," she said.

Dr. Bond and Ms. Whitehead have disclosed no relevant financial relationships.

53rd Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC): Abstracts D-113 and D-114. Presented September 10, 2013.


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