Legionella Urinary Antigen

The Legionella urinary antigen test detects the soluble lipopolysaccharide antigen of Legionella pneumophila serogroup 1 excreted in urine, by lateral flow immunochromatography or enzyme immunoassay. It is the commonest first line test for Legionnaires disease, with results in 15 minutes to 3 hours, and detects antigen from 1 to 3 days after symptom onset.

Sensitivity is 70 to 80% overall (up to 90% in severe disease) for serogroup 1, which causes 70 to 90% of community acquired Legionnaires disease in Europe and North America, with specificity of 95 to 100%. It does not detect other serogroups or species reliably (Legionella longbeachae, L. micdadei, L. bozemanii), which cause up to 30% of cases in some regions such as Australia and New Zealand.

Legionnaires disease accounts for 2 to 9% of community acquired pneumonia needing hospital admission, and travel associated, hospital acquired and outbreak related clusters are frequent. Transmission is by inhaled aerosols from contaminated water systems (cooling towers, hot tubs, showers, spas, humidifiers).

Mortality is 5 to 10% in community acquired disease and up to 25 to 50% in hospital acquired disease or when treatment is delayed. Risk factors are age over 50 years, smoking, chronic lung disease, immunosuppression, diabetes, alcohol excess and renal failure.

Indications. Severe community acquired pneumonia needing intensive care or hospital admission. Pneumonia with extrapulmonary features (hyponatraemia, diarrhoea, confusion, raised liver enzymes, raised creatine kinase, haematuria) or not responding to beta lactam antibiotics. Pneumonia after travel, hotel or cruise stays, spa exposure, or recent exposure to a water system.

Hospital acquired pneumonia in transplant or immunosuppressed patients. Suspected outbreak or cluster. Not for monitoring.

Specimen. Random urine 1 to 5 mL, collected in a clean container. Concentration of urine by ultrafiltration increases sensitivity by about 10%. Stable at room temperature for 24 hours and at 2 to 8 °C for 14 days. Collect together with a respiratory sample (sputum, endotracheal aspirate or bronchoalveolar lavage) for culture on buffered charcoal yeast extract agar and PCR, because the urinary test alone gives no isolate for epidemiological typing.

Reference values. Negative (no antigen detected). A positive is a visible test line at the correct time.

Interpretation.

• Positive result with compatible pneumonia: Legionnaires disease due to L. pneumophila serogroup 1 is very likely (positive predictive value over 95%). Start or confirm macrolide, fluoroquinolone or doxycycline treatment, and send respiratory samples for culture and typing to link cases.
• Negative result: does not exclude Legionnaires disease (negative predictive value 90 to 95% in low prevalence, lower in non-serogroup 1 infection and early disease). Send respiratory PCR and culture when suspicion remains.
• Early sample (under 2 days from onset) may be negative, and a repeat test after 24 to 48 hours has a higher yield.
• Antigen excretion can persist for weeks to months after infection (median 2 months, up to a year) in some patients, so a positive result in a patient with a previous episode may reflect old infection.
• Positive urinary antigen with negative culture and PCR: serogroup 1 infection, with no isolate for comparison with environmental sources. Efforts to obtain a lower respiratory sample are needed in outbreaks.
• Legionella PCR on lower respiratory samples (sensitivity 90 to 95% for all species) covers non-serogroup 1 species.
• Seroconversion (fourfold rise in antibody titre over 3 to 6 weeks) is retrospective and of epidemiological use.
• Associated findings that raise probability: sodium under 130 mmol/L, raised C-reactive protein over 300 mg/L, lymphopenia, raised lactate dehydrogenase and creatine kinase, abnormal liver tests, and patchy or lobar consolidation worsening despite beta lactam therapy.

Critical values and findings needing immediate action.

• Positive antigen in a severely ill patient: intensive care review, intravenous levofloxacin or azithromycin, and consideration of combination therapy in severe disease.
• Positive antigen in a hospital inpatient, immunosuppressed or recent travel patient: notify infection control and public health, and investigate the water systems.
• Cluster of cases: urgent public health notification and environmental sampling.
• Pneumonia with respiratory failure, shock, confusion, renal failure or rhabdomyolysis: organ support and early specific treatment, without waiting for confirmation.
• Negative antigen with a strong clinical picture and poor response to beta lactam therapy: continue Legionella active treatment while PCR and culture are pending.

Interferences and pitfalls.

• The test detects only L. pneumophila serogroup 1 reliably, so it misses disease due to other serogroups and species, particularly in immunosuppressed patients and in regions where L. longbeachae is common.
• Sensitivity is lowest in mild disease and early illness, and false negatives occur if the urine is dilute.
• Cross-reactions are rare, with occasional false positives from other pathogens (for example, Mycoplasma), rheumatoid factor or high protein urine.
• Prolonged positivity can mislead in recurrent pneumonia.
• Rapid tests give no isolate, and culture of lower respiratory samples (3 to 7 days) remains important for outbreaks, with results influencing public health action.
• Heating urine specimens (boiling for 5 minutes) reduces false positives in enzyme immunoassay in some systems.
• Beta lactams and aminoglycosides are inactive against Legionella, because the organism is intracellular.
• Interactions occur between macrolides or fluoroquinolones and drugs prolonging the QT interval, and between macrolides and ciclosporin or tacrolimus.
• Not all cases require antigen testing, and local policy balances yield and cost.

Special groups.

• Older adults and smokers: most cases, with severe disease.
• Transplant recipients and patients on biologic or corticosteroid therapy: more severe disease and non-pneumophila species.
• Children: rare, mostly with immunodeficiency.
• Pregnancy: use of azithromycin or erythromycin rather than fluoroquinolones or doxycycline.
• Regions: L. longbeachae from potting compost and soil in Australia, New Zealand and parts of Asia. Outbreaks in hotels, cruise ships and hospitals are linked to ageing water systems and warm climates.

Follow up tests. Sputum or lower respiratory sample for Legionella culture and PCR, respiratory PCR panel for other pathogens, blood cultures, serum sodium, liver enzymes, creatine kinase, creatinine and C-reactive protein, chest radiograph or CT, paired serology, pneumococcal urinary antigen, arterial blood gas, ECG for QT interval, whole genome typing of clinical and environmental isolates in outbreaks, and water system sampling.