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Invasive aspergillosis (IA) still remains difficult to diagnose and hard to
treat in immunocompromised patients. The mortality rate due to this
life-threatening opportunistic mycosis is as high as 50-100% and early
diagnosis and prompt initiation of antifungal therapy remain as the crucial
factors that may aid in reduction of the mortality rates. Definitive
diagnosis of IA by cultivation of the infecting strain requires surgical
biopsy of a sterile site for optimal results, may take as long as 4 weeks,
and may be confounded by contamination. Although biopsy and culture will
doubtless remain as gold standard tests, there is a remarkable demand for
additional tests that may help in early diagnosis of this infection.
Among these, detection of galactomannan antigen in serum and body fluids
has been intensively studied in recent years. Galactomannan is a component
of the fungal cell wall and an exoantigen of Aspergillus. The first test to detect this
antigen used a latex agglutination (LA) method. Subsequently, an
ELISA-based method was developed which provided higher sensitivity and
specificity compared to LA [2125].
The ELISA-based kit is commercially available as "Platelia Aspergillus EIA" (Bio-Rad Laboratories).
Galactomannan antigen positivity is among the microbiological diagnostic
criteria proposed by European Organization for Research and Treatment of
Cancer (EORTC) and Invasive Fungal Infections Cooperative Group and the
National Institute of Allergy and Infectious Diseases Mycoses Study Group
(MSG) for diagnosis of IA. The kit has been available for some time in
Europe and was cleared for diagnostic use in the United States by the FDA
on 16 May 2003. The cut-off used in the European kit was an index of 1.0
for indeterminate and 1.5 for positive. The FDA cleared version of the test
kit uses a cut-off of 0.5 index value and demonstrated good sensitivity and
specificity claims in the clinical testing from three separate medical
centers.
Here is a summary of the key data regarding use of the galactomannan
antigen test in the diagnosis of IA:
- The test can detect as little as 0.5-1
ng/ml of galactomannan in the tested sample [2003].
- The most frequently tested sample and the
one which provides highest sensitivity is serum [1840].
Other samples, such as bronchoalveolar lavage, urine [1840]
and cerebrospinal fluid [2123,
2139]
may also be beneficial in diagnosis of IA, but the kit is standardized
and FDA cleared only for serum samples. The Platelia Aspergillus EIA
has not been evaluated for use with plasma or other sample types such
as urine, BAL or CSF in the US. In addition, the performance of the
Platelia Aspergillus EIA has not been evaluated in neonate or
pediatric serum samples in the US.
- The galactomannan antigen test is a
screening test and should be employed as such. Collection of serum
samples should be initiated right after hospitalization in patients at
high risk of developing IA. Samples should be collected (at least)
twice weekly from then on [1284].
The concomitant use of mould-active antifungal therapy in some
patients with IA may result in reduced sensitivity with the Platelia
Aspergillus EIA.
- Detection of positive results particularly
in two consecutive serum samples provides strong support for the
diagnosis of IA [1283,
1284].
The US package insert recommends that an initially positive sample be
re-tested by performing a treatment of a new aliquot of the serum
sample and also collecting another sample for testing. Samples should
be reported as negative or positive for galactomannan. The index value
for positive samples can prove to be useful. Negative samples should
just be reported as negative. A negative test cannot rule out the
diagnosis of IA. Repeat testing of additional samples should be
considered where there is clinical suspicion of IA or a procedural
error.
- Galactomannan detection can provide two
types of information. First, early diagnosis is possible if the
specimens are being tested upon collection. Galactomannan antigen
positivity can be detected 5-8 days (average) before clinical signs
develop (in 65.2% of patients), findings on chest X-ray are visible
(in 71.5% of patients) and culture results become positive (in 100% of
patients) [1283].
A second use is to provide negative data in patients with syndromes
compatible with aspergillosis. In such settings, the absence of
galactomannan positivity may provide support for more aggressive
traditional diagnostic measures.
- The sensitivity and specificity of the
test are favorable. In the dataset evaluated by FDA, the overall
sensitivity and specificity of the method were 80.7% and 89.2%,
respectively. For this evaluation, 1890 blood samples collected from
170 patients were tested. Among these, 31 patients had proven or
probable IA. Galactomannan antigen was detected in 25 of these 31
patients with a resulting sensitivity of 80.7%. Of 148 patients who
did not have IA, 132 were negative for galactomannan antigen, yielding
a specificity of 89.2%. Of these 148 patients, a total of 1362 sera
were obtained and tested. 1343 of these 1362 sera were initally
negative, resulting in a sample agreement of 98.6% with a 95% CI of
97.9-99.3. On repeat testing, 1355 of the 1362 sera were negative.
- As with any serological test, false
positive reactions may be observed. It has been detected in 1-18% of
the tested samples [2264,
1284].
False positivity may originate from true antigenic cross reactivity
with other fungi such as Penicillium chrysogenum, Penicillium digitatum, and Paecilomyces variotii [2024].
A pattern of false-positive antigenemia that is most common in
children [1204]
has been noted and may possibly develop after translocation of
galactomannan antigen found in various foodstuffs (unsalted bread,
macaroni, corn flakes, salted rice, dry cake, turkey slices, grilled
sausage, fried potatoes, etc.) through the damaged intestinal mucosa [1204].
- As well as early diagnosis, the test may
also be helpful in assessment of the prognosis and the clinical
response to antifungal therapy [1583,
1788,
283].
In cases of a favorable clinical response, the titer of the antigen
tends to either decline or does not change significantly compared to
the baseline titer. In contrary, it increases significantly in case of
treatment failure [1840,
2123,
283].
Of note, and possibly due to the distinctive mode of action of the
drug, galactomannan antigenemia persisted despite clinical improvement
in animals who were treated with caspofungin [1609].
This finding suggests that same may apply to patients who are treated
with an echinocandin compound.
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