Laboratory definitive evidence for a confirmed case requires at least one of the following:

• isolation of leptospires from a clinical specimen
• detection of leptospiral nucleic acid from a clinical specimen
• a four-fold or greater rise in leptospiral microscopic agglutination titre (MAT) between acute and convalescent sera
• single high antibody titre of ≥ 400 in the MAT

Laboratory suggestive evidence for a probable case requires single raised agglutination titre by MAT of <400.

It is recommended that both nucleic acid testing (NAT) and MAT testing be undertaken to improve diagnostic accuracy. MAT is the current gold standard serological test and is used to identify the probable causative serovar/serogroup. ESR-NCBID is the national reference laboratory for MAT.

Serology: IgM can be detectable within the first week of illness and can persist for months. Seroconversion can take up to 3 weeks from the onset of symptoms. IgM is useful as a screening test but not a confirmatory test because of potential cross-reactivity with other diseases. For confirmatory testing acute and convalescent samples need to be tested in parallel by MAT. There should be a minimum of 2 weeks between collection of acute and convalescent sera.

Nucleic acid testing (for example, polymerase chain reaction – PCR): NAT is highly sensitivity for diagnosis of leptospirosis. NAT can be used to detect leptospires in blood during the acute leptospiraemic phase of the disease typically before an antibody response is mounted. NAT can also be used to detect leptospires in urine during the second week of illness; shedding may be prolonged and intermittent.

Leptospires can be excreted intermittently in the urine. Therefore, a negative result in the context of a compatible clinical illness cannot exclude the diagnosis of leptospirosis.

In cases of high clinical suspicion, a second urine sample should be submitted if the initial specimen tested negative by NAT.

Please refer to the Ministry of Health Communicable Disease Control Manual for the most up to date leptospirosis lab criteria.