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Multicenter Evaluation of a Nonweekend Reading Schedule for Radiometric Pyrazinamide Susceptibility Testing of Mycobacterium tuberculosis

B. Madison,^1* W. Gross,² I. George,³ A. Sloutsky,³ G. Washabaugh,⁴ B. Robinson-Dunn,^5, H. Lipman,¹ B. Metchock,¹ G. Mazurek,¹ and J. Ridderhof¹

Centers for Disease Control and Prevention, Atlanta, Georgia,¹ Veterans Administration TB Reference Laboratory, West Haven, Connecticut,² State Laboratory Institute, Boston, Massachusetts,³ San Diego County Public Health Laboratory, San Diego, California,⁴ Michigan Department of Community Health, Lansing, Michigan⁵

Received 12 April 2002/ Returned for modification 18 June 2002/ Accepted 20 July 2002

Pyrazinamide (PZA) is an integral component of the short-course chemotherapy regimen for tuberculosis. The BACTEC 460TB PZA susceptibility test for Mycobacterium tuberculosis with a daily (D) reading schedule has been available for more than 10 years, but weekend laboratory staffing is necessary. A nonweekend (NW) reading schedule has not been validated in a multicenter study. This prospective multicenter study compares the interlaboratory reproducibility of PZA susceptibility results by following both the D and NW schedules. A total of 181 cultures were shared among four laboratories. Isolates were selected based on resistance or borderline resistance to at least one streptomycin-isoniazid-rifampin-ethambutol drug or PZA. One laboratory used a D reading schedule, and three laboratories used a NW schedule. Both reading schedules are based on the standard BACTEC 460TB PZA protocol. With the NW schedule, the growth index (GI) is not available for test interpretation on Saturday, Sunday, and Monday. Of the 181 shared cultures, 154 were found to be susceptible by all laboratories, 19 were found to be resistant, and 8 had discordant results. The overall pairwise interlaboratory agreement was 97.7%. The discrepancies were not associated with the type of reading schedule used. However, the median control GI was significantly higher for the NW schedule (321) than for the D schedule (259) (P < 0.0001) although results were available on average in about 7 days from setup for both schedules. These results show that the NW schedule is a suitable alternative for laboratories that do not read and interpret PZA susceptibility tests on weekends.

Present address: Clinical Pathology Department, William Beaumont Hospital, Royal Oak, MI 48073.