Rapid, Easy, and Reliable Identification of Nocardia sp. by MALDI-TOF Mass Spectrometry, VITEK®-MS IVD V3.2 Database, Using Direct Deposit

The reference methods for Nocardia identification are based on gene sequencing. These methods are time-consuming and not accessible for all laboratories. Conversely, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry is easy to use and widely available in clinical laboratories, but for Nocardia identification, the VITEK (R)-MS manufacturer recommends a tedious step of colony preparation that is difficult to integrate into a laboratory workflow. This study aimed to evaluate Nocardia identification by MALDI-TOF VITEK (R)-MS using direct deposit with the VITEK (R)-PICKME (TM) pen and a formic acid-based protein extraction directly onto the bacterial smear on a 134 isolates collection; this identification was compared to the results from molecular reference methods. For 81.3% of the isolates, VITEK (R)-MS delivered an interpretable result. The overall agreement with the reference method was 78.4%. Taking only the species included in the VITEK (R)-MS in vitro diagnostic V3.2 database into account, the overall agreement was significantly higher, 93.7%. VITEK (R)-MS rarely misidentified isolates (4/134, 3%). Among the 25 isolates that produced no result with the VITEK (R)-MS, 18 were expected, as Nocardia species were not included in the VITEK (R)-MS V3.2 database. A rapid and reliable Nocardia identification using direct deposit by VITEK (R)-MS is possible by combining the use of the VITEK (R)-PICKME (TM) pen and a formic acid-based protein extractiondirectly onto the bacterial smear.

Automated summary

This study evaluated Nocardia identification using MALDI-TOF VITEK(R)-MS with direct deposit. The overall agreement with the reference method was 78.4%. Excluding Nocardia species not included in the VITEK(R)-MS database, the overall agreement significantly improved to 93.7%. VITEK(R)-MS rarely misidentified isolates (4/134, 3%), and among the 25 isolates that produced no result, 18 were expected due to the absence of the species in the VITEK(R)-MS database.