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Combine qPCR and GM antigen tests for maximum sensitivity/specificity in disseminated Talaromycosis

September 16 2019

Talaromyces (formerly Penicillium) marneffei can cause disseminated disease in immunocompromised patients. Clinical signs are non-specific and identification by culture can take 1-4 weeks, by which time the patient may be dead.

Previously, qPCR has been shown to detect Talaromyces DNA in whole blood and plasma. Dr Xinlei Li and colleagues at Guangxi Medical University (China) tested a novel TaqMan qPCR assay based on ITS1-5.8S- ITS2 rDNA. They compared serum (which is easier to test/store than blood) from patients with culture-confirmed talaromycosis (n=36), invasive aspergillosis (n=34), cryptococcosis (n=11), or no fungal infection (n=30).

The qPCR assay was positive for 20/20 samples from talaromycosis patients whose blood was culture positive, and also positive for 11/16 whose blood was culture negative. Median Cq values were 34 and 36 cycles respectively. Overall, sensitivity of the qPCR assay was 86%, versus only 56% for conventional blood culture. None of the samples from patients with aspergillosis/cryptococcosis/no fungal infection tested positive (i.e. specificity was 100%).

Other studies have found the galactomannan antigen test to be useful for diagnosing talaromycosis in patients with HIV/AIDS. The group compared their qPCR assay to the Platelia Aspergillus enzyme immunoassay (see table). These results suggest that talaromycosis should be added to the differential diagnosis in patients with elevated galactomannan antigen in areas where the infection is endemic (southeast Asia, see map below), especially for HIV-positive patients. Where facilities are available, GM and qPCR can be combined to achieve high specificity and sensitivity.

 

SERUM galactomannan OD index

Lower range

Median

Upper range

Healthy

0.12

0.43

0.82

Talaromycosis

0.15

1.31

7.22

Aspergillosis

0.22

2.31

7.41

Cryptococcosis

0.15

0.24

1.2

 

†   These (mostly HIV-negative) patients had culture-negative blood samples, but had other types of sample that were culture positive.