Cautions

Results from the Xpert SA Nasal Complete Assay should be interpreted in conjunction with other laboratory and clinical data available to the clinician and should be used as an adjunct to nosocomial infection control efforts to identify patients needing enhanced precautions. Results should not be used to guide or monitor treatment for methicillin-resistant Staphylococcus aureus (MRSA) or S aureus infections.

Erroneous test results might occur from improper specimen collection, failure to follow the recommended specimen collection, handling and storage procedures, technical error, sample mix-up, or because the number of organisms in the specimen is too low to be detected by the test.

A positive test result does not necessarily indicate the presence of viable organisms. It is, however, presumptive for the presence of S aureus or MRSA.

An Xpert SA Nasal Complete Assay positive result does not necessarily indicate intervention eradication failure since nonviable DNA may persist. A negative result following a previously positive test result may or may not indicate eradication success.

A negative result does not negate the presence of MRSA or S aureus.

Concomitant cultures are necessary to recover organisms for epidemiological typing or for further susceptibility testing.

The performance characteristics were not established by Cepheid for patients 21 years of age or younger.

Mutations or polymorphisms in primer or probe binding regions may affect detection of new or unknown S aureus or MRSA variants resulting in a false-negative result. 

In samples containing both MRSA and methicillin-susceptible S aureus, the Xpert SA Nasal Complete Assay may not detect the MRSA organisms. The pivotal clinical study included one sample with documented mixed infection; the Xpert SA Nasal Complete Assay identified the sample as MRSA positive/S aureus positive.

In a mixed culture, the analytical limit of MRSA detection is variable when extremely high concentrations of S aureus are present. Competition from S aureus was observed at a MRSA: S aureus ratio of 1:1x10(6) in 7 of 8 SCCmec types tested. For SCCmec type VIII, competition from SA was observed at a MRSA: S aureus ratio of 1:1x10(3).

Inhibition of the SA Nasal Complete Assay resulting in invalid test results has been observed in the presence of inhaled nasal steroids Flonase and Nasonex in SA negative samples at concentrations greater than 5% (volume to volume [v/v}), and 10% (v/v), respectively.

Inhibition of the SA Nasal Complete Assay resulting in false-negative test results has been observed in the presence of inhaled nasal steroids Flonase and Nasonex in MRSA positive samples at concentrations greater than 1% (v/v) and 5% (v/v), respectively.

The Xpert SA Nasal Complete Assay may generate a false-positive MRSA result when testing a mixed infection nasal specimen containing both methicillin-resistant coagulase-negative Staphylococcus species and empty cassette S aureus.

The Xpert SA Nasal Complete Assay may generate false-negative MRSA results when testing borderline oxacillin-resistant S aureus (BORSA). The mechanism of oxacillin resistance in BORSA strains is due to an increased production of beta-lactamases, not mecA. BORSA with oxacillin minimum inhibitory concentrations (MIC) of 4 to 8 mcg/mL are considered borderline resistant but would be reported as MRSA negative by the Xpert SA Nasal Complete Assay. BORSA strains are rare in the United States.

The Xpert SA Nasal Complete Assay may generate false-negative MRSA results when testing modified S aureus (MOD-SA). The mechanism of oxacillin resistance in MOD-SA strains is due to changes in affinity of penicillin binding proteins for oxacillin, not mecA. MOD-SA with oxacillin MIC of 4 to 8 mcg/mL are considered borderline resistant but would be reported as MRSA negative by the Xpert SA Nasal Complete Assay. MOD-SA strains are rare in the United States.

There may be an association with false-positive results in specimens containing blood.

Xpert SA Nasal Complete assay results may sometimes be invalid due to a failed SPC control, error, or no result and require retesting, which can lead to a delay in obtaining final results.

As with all in vitro diagnostic tests, positive and negative predictive values are highly dependent on prevalence. Xpert SA Nasal Complete assay performance may vary depending on the prevalence and population tested.(1)