Synthetic nucleic acid spike-ins

A method for identifying carry-over between samples using a sample identifier spike-in, the method comprising:

• (a) providing a first sample comprising a first microbial nucleic acid from a first subject;
• (b) providing a second sample comprising a second microbial nucleic acid from a second subject;
• (c) providing a second-sample identifier spike-in, wherein
• the second-sample identifier spike-in comprises a synthetic nucleic acid and
• does not comprise sequences capable of hybridizing to the first microbial nucleic acid or the second microbial nucleic acid;
• (d) adding the second-sample identifier spike-in to the second sample, wherein a number of molecules of the synthetic nucleic acid of the second-sample identifier spike-in is equal to or greater than a number of molecules of the second microbial nucleic acid in the second sample;
• (e) assaying the first sample to detect a presence, an absence, or an amount of the second-sample identifier spike-in and the first microbial nucleic acid;
• and
• (f) determining that the first microbial nucleic acid was obtained from the first subject based at least upon, in (e), detection of:
• i. the first microbial nucleic acid;
• and
• ii. the absence of the second-sample identifier spike-in or
• the amount of the second-sample identifier spike-in below a threshold level.