Sensitivity, specificity and predictive values of anterior chamber tap in cases of bacterial endophthalmitis
Sjoholm-Gomez de Liano et al. Eye and Vision
Sensitivity, specificity and predictive values of anterior chamber tap in cases of bacterial endophthalmitis
Carl Sjoholm-Gomez de Liano 1 2 4
Vidal F. Soberon-Ventura 1 2 4
Guillermo Salcedo-Villanueva 1 2 4
Abril Santos-Palacios 2 3
Jose Luis Guerrero-Naranjo 0 1 2 4
Jans Fromow-Guerra 0 1 2 4
Gerardo García-Aguirre 0 1 2 4
Virgilio Morales-Canton 1 2 4
Raul Velez-Montoya 0 1 2 4
0 Ophthalmology Department , Macula Retina Consultants, Mexico City , Mexico
1 Retina Department, Asociación para Evitar la Ceguera en Mexico, Hospital “Dr. Luis Sanchez Bulnes” IAP , Mexico City , Mexico
2 The manuscript was presented in a preliminary version, as a poster in the Mexican Association of Retina Specialist annual meeting , 2016
3 Universidad Popular Autonoma del Estado de Puebla , Puebla , Mexico
4 Retina Department, Asociación para Evitar la Ceguera en Mexico, Hospital “Dr. Luis Sanchez Bulnes” IAP , Mexico City , Mexico
Background: To assess the sensitivity, specificity, positive predictive value and negative predictive value of anterior chamber tap for the diagnosis of bacterial endophthalmitis on a population with high prevalence. Methods: Retrospective, single centre, case series study. We reviewed all medical records with clinical diagnosis of bacterial endophthalmitis in our hospital from January 1st, 2000 to December 31st 2014. From each record, we documented general demographic data, best corrected visual acuity and vitreous and aqueous tap microbiological results. All cases were further divided according to the endophthalmitis aetiology to perform individual calculations of sensitivity, specificity, positive predictive value, negative predictive value, accuracy and prevalence. We used the results of the vitreous tap as the gold standard for diagnosis of bacterial endophthalmitis. We excluded those records in which the aqueous and vitreous samples were not taken simultaneously or had an incomplete microbiological report. Significance were assessed with chi squared statistics, with an alpha value of 0.05 for statistical significance. Results: A total of 190 cases fulfilled the inclusion/exclusion criteria. Positive culture rate from vitreous samples was 64.74%. Positive culture rate from aqueous sample was 32.11%. Bacteria isolated from aqueous samples matched those isolated from vitreous samples 78.68% of the time. The overall sensitivity was 38.21%, specificity: 75.51%, positive predictive value: 79.66%, negative predictive value: 32.74% (p = 0.08). Subgroup analysis showed that anterior chamber taps in cases of post-surgical endophthalmitis had a moderate to low sensitivity (37.73%), high specificity (93%) and high positive predictive value (95%) (p < 0.04). Conclusion: The sensitivity and specificity of anterior chamber tap are low and should not be used for critical therapeutic decisions in patients with suspected bacterial endophthalmitis. In cases of post-surgical endophthalmitis, the result of an anterior chamber tap could be used for therapeutic guidance, but only in conjunction with clinical presentation and in the absence of a better method for diagnosis.
Aqueous humour; Endophthalmitis; Anterior chamber tap; Diagnostic test; Vitreous tap; Aqueous sampling; Sensibility; Sensitivity
The accurate diagnosis of endophthalmitis, along with
the precise identification of the microorganism involved,
is crucial for achieving good treatment outcome. Due to
the accessibility of the anterior chamber, the diagnostic
workup of endophthalmitis in many centres includes an
aqueous tap; the result may be used for making
therapeutic decisions. Our study shows that its overall
sensitivity, specificity, positive predictive value and negative
predictive value were low to moderate and not
significant. However, subgroup analysis showed that cases of
post-surgical endophthalmitis had a high specificity and
high predictive value (p < 0.04). Therefore, it could be
used as a confirmatory test in such cases, but only in
populations with a high prevalence (80%).
Endophthalmitis is a rare but potentially blinding
condition characterized by the colonization, inflammation and
irreversible destruction of intraocular tissue by an
infectious agent. Clinical outcomes are closely related to several
factors including aetiology, the source of contamination,
virulence of the pathogen and antibiotic sensitivity, time of
evolution, and treatment delays among others [
Although general guidelines for the treatment of bacterial
infections include pathogen identification and selective
antibiotic therapy, the high risk of permanent visual loss,
along with the lack of a fast and reliable screening tests, a
strong clinical suspicion could justify for the immediate
empirical treatment with broad-spectrum intravitreal
Nevertheless, as part of the general workup and before
intravitreal antibiotics, a vitreous tap is still attempted to
guide therapeutic decision in the future, especially if the
treatment response is suboptimal and to confirm the
clinical diagnosis [
A successful vitreous tap poses several technical
challenges. For instance, the patient is probably in severe
pain during sample collection and therefore
uncooperative; the relatively high volume of the vitreous cavity and
the vitreous viscosity preclude the collection of an
adequate and sufficient vitreous sample most of the time.
Moreover, a sufficient but small sample may decrease
the possibility of a successful bacterial growth in culture
and increase the false negative rate of the test (30–40%
of vitreous samples are culture negative) [
The anterior chamber is an anterior, highly exposed
and easily accessed intraocular space, filled with aqueous
humour; a clear fluid mainly composed of water, which
is in close relation with the vitreous through the
posterior chamber and zonules [
]. Due to the
uniqueness of this anatomical relationship, anterior chamber
taps might be included as part of the regular
endophthalmitis workup as complementary tests and could be
used for making treatment decisions in the absence of a
positive vitreous sample [
12, 15, 16
]. Therefore, the aim
of the following study is to assess the concordance
between positive cultures from aqueous and vitreous
samples in patients with bacterial endophthalmitis and to
establish the sensitivity, specificity and predictive values
that aqueous sampling has as a diagnostic test for
Retrospective, single centre, case series study. The study
was approved by the hospital’s Internal Review Board.
The study was conducted according to the tenets of the
declaration of Helsinki and good clinical practices
guidelines. All sensitive data were managed according to the
Health Insurance Portability and Accountability Act
(HIPAA) rules of 1996 and the Mexican Federal Law for
Protection of Personal Data in Possession of Individuals
(NOM-024-SSA3–2010). Due to its retrospective nature,
no informed consent was needed.
We reviewed all medical records with clinical
diagnosis of acute infectious endophthalmitis between January
1st, 2000 and December 31st, 2014. The diagnosis of
acute infectious endophthalmitis was performed according
to clinical presentation (3 to 5 days to onset, severe visual
loss, severe ocular pain, severe conjunctival hyperaemia,
anterior chamber and vitreous inflammation [aqueous
cells, anterior chamber hypopyon, vitreous cells and
opacification]) and suggestive ophthalmic B-scan
ultrasonography. Files from patients with suspected toxic anterior
segment syndrome (either by an unusually quick onset of
symptomatology, generalized corneal oedema or
ambiguous symptomatology) were excluded.
We included only those records in which an aqueous
and vitreous sample were drawn and analysed
simultaneously during the initial endophthalmitis workup, and
had a complete report from the microbiology
department (gram stains, bacterial cultures [blood agar,
chocolate agar, MacConkey agar and Sabouraud agar], strain
identification and antibiotic sensitivity tests). All
analysed samples were of undiluted vitreous and aqueous,
obtained by fine needle aspiration before intravitreal
antibiotics. All files in which the sample was classified as
insufficient were excluded.
We also excluded all incomplete medical records
where the initial suspicion (due to the medical history or
clinical presentation) was fungal, viral or polymicrobial
endophthalmitis. We also excluded cases with high
suspicion of pseudo-endophthalmitis and cases where a
non-biological agent was suspected to be responsible for
the clinical presentation of severe intraocular
inflammation. Finally, we excluded all medical records where the
patients were receiving antibiotic treatment (topical or
intravitreal) previous to aqueous and vitreous sampling.
From each medical record, we documented the age
and gender of the patient and laterality of
endophthalmitis (OD: right eye, OS: left eye). Best corrected visual
acuity (BCVA) assessments were converted from Snellen
charts to its logarithmic minimum angle of resolution
equivalent (logMAR) for statistical analysis. Visual
acuities of count fingers (CF) equated to 1.7 logMAR; hand
movement (HM) to 2.0 logMAR; light perception (LP)
to 2.3 logMAR and no light perception (NLP) to 3.0
All cases were further divided into six groups
according to the aetiology (source of infection) in order to
perform individual calculations for sensitivity, specificity,
positive predictive value and negative predictive value
for each group: 1) post-surgical endophthalmitis; 2)
post-traumatic endophthalmitis; 3) endophthalmitis
associated to corneal ulcers; 4) endogenous
endophthalmitis; 5) post-intravitreal injections endophthalmitis and 6)
endophthalmitis associated to glaucoma procedures.
From the microbiology department report, we
documented the genus and species of the isolated pathogen
of the vitreous and aqueous samples. A positive vitreous
sample is the gold standard for the diagnosis of
endophthalmitis. A positive result meant that a pathogen was
successfully grown in culture and characterized
accordingly. A negative result meant that no growth was
possible and the gram stain was negative as well.
Therefore, a negative aqueous sample with a positive vitreous
sample was considered to be a false negative (FN); a
positive aqueous sample with a positive vitreous sample
was considered to be a true positive (TP), as long as
both samples yielded the same pathogen. If the aqueous
sample reported a different pathogen than the vitreous
sample, it was a false positive result (FP). Only cases
with negative aqueous and vitreous samples were true
negative results (TN).
Aqueous sample sensitivity (SNaq) was calculated as
the function of TP / (TP + FN). Aqueous sample
specificity (SPaq) was calculated as the function of TN /
(TN + FP). The positive predictive value of the aqueous
sample (PPVaq) was calculated as the function of TP /
(TP + FP). The negative predictive value of the aqueous
sample (NPVaq) was calculated as the function of TN
/ (TN + FN). Finally, the accuracy (ACCaq) of the
test was calculated as the function of (TP + TN) /
(TP + FP + FN + TN).
Statistical analysis was performed using Microsoft
Excel (Excel 2010; Microsoft Corp., Redmond, WA) with
an XLSTAT application v18.06 (Addinsoft, New York,
NY). Changes in BCVA were assessed with a Wilcoxon
two sample test, with an alpha value of 0.05 or less for
statistical significance. A one way ANOVA test was used
to identify differences in the variability of the means of
patient’s age and BCVA among aetiology groups with an
alpha value of less than 0.05 for statistical significance. A
Fisher unprotected least significant difference test was
used to assess statistical differences between means
within study groups. Significance of the SPaq, SNaq,
PPVaq, NPVaq and ACCaq was assessed with Chi
squared statistics with an alpha value of 0.05 for
A total of 190 medical records, who fulfilled the
inclusion and exclusion criteria, were included in the study.
Overall, there were 114 males and 76 females; 93 OD
and 95 OS and in two cases the diagnosis was bilateral.
The mean age at presentation was 55.14 ± 20.9 years. In
54 cases (28.42%), the diagnosis of endophthalmitis was
suspected as a direct result of medical attention received
in our hospital. The 136 additional cases were sent to us
from different ophthalmology clinics and private offices
from the community. According to the aetiology, 74 cases
were classified as post-surgical cases of endophthalmitis
(post phacoemulsification), 42 cases were classified as
post-traumatic endophthalmitis, in 26 cases, the diagnosis
of endophthalmitis was associated with corneal ulcers.
Thirteen cases were classified as endogenous
endophthalmitis. Thirty-two cases were classified as endophthalmitis
due to intravitreal injections. The remaining three cases
were late onset endophthalmitis due to previous glaucoma
procedures. Table 1 summarizes the mean age at
presentation, gender dominance, laterality and BCVA, according
to the different aetiology groups. Patients with
posttraumatic endophthalmitis were significantly younger than
the patients within other groups (p < 0.001). Patients with
post-surgical endophthalmitis showed better clinical
outcomes with significant improvement of BCVA
(p < 0.05). The other groups had a trend toward
improvement, but the change in BCVA did not achieve
statistical significance (Fig. 1).
Overall, in a total population of 190 cases, the
prevalence of vitreous sample positive-endophthalmitis
(culture positive endophthalmitis) in the study was 64.74%
(7 out of 10 cases were eventually confirmed as
endophthalmitis by vitreous gram stain and bacterial cultures
growth). Regarding the aqueous samples, only 61 out of
190 samples yielded a positive bacteria identification by
gram stain and bacterial culture growth (positive culture
rate: 32.11%). In 48 cases of the 61 positive aqueous
samples (78.68%), the pathogen isolated matched the
bacteriological results from the vitreous sample drawn
from the same eye.
Staphylococcus epidermidis was the most common
pathogen isolated from vitreous samples, in patients with
post-surgical endophthalmitis (23 cases; 31.08%);
posttraumatic endophthalmitis (9 cases, 21.43%) and
postintravitreal injection endophthalmitis (7 cases, 21.88%).
(years ± SD)
Abbreviations: BCVA = best corrected visual acuity, SD = Standard deviation, M = male, F = Female, OD = right eye, OS = left eye, OU = both eyes
astatistically significant data
The most commonly isolated pathogen in cases of
endophthalmitis associated to corneal ulcers was
Streptococcus pneumoniae (15.38%). In cases of endogenous
endophthalmitis, the predominant pathogen was Candida
spp. (23.07%). All three cases of delayed endophthalmitis
due to glaucoma procedures were caused by gram
negative pathogens (Serratia marcescens, Moraxella
nonliquefaciens and Haemophilus influenzae).
Despite most of endogenous endophthalmitis included
in this study being bacterial in origin, there were three
cases of Candida spp. Therefore, the change in BCVA on
this particular group may not mirror a precise response to
treatment since they were treated first with intravitreal
antibiotics. Intravitreal voriconazole was used only after
hyphae identification or primary failure of the treatment.
Table 2 summarizes the SNaq, SPaq, PPVaq, NPVaq
and ACCaq, according to endophthalmitis aetiology.
Only in post-surgical cases of endophthalmitis, aqueous
sampling had a statistically significant high specificity
(93%), and positive predictive value (95%) for a subgroup
endophthalmitis prevalence of 79.03%. Calculation could
not be made in cases of delayed endophthalmitis due to
glaucoma procedures because of insufficient sample size.
Clinical and anatomical outcomes after acute bacterial
endophthalmitis remain highly dependent on prompt
diagnosis and appropriate instalment of the treatment
]. Although the gold standard for diagnosis needs the
definitive identification of a bacteria from a vitreous
sample, many technical difficulties might be encountered
during its collection. Vitreous viscosity may decrease the
capability of the examiner to draw enough sample for
appropriate laboratory processing. In addition, scleral
rigidity, elevated intraocular pressure and severe local
inflammation/pain may decrease the chances of a
successful test [
2, 11, 12
]. Therefore, anterior chamber taps have
been proposed as an alternative tool for diagnosis, mainly
due to the good accessibility of the anterior chamber and
higher probability of adequate sampling [
8, 9, 12
Previous studies have questioned the diagnostic value of
anterior chamber taps in cases of bacterial
endophthalmitis. In clinical and preclinical studies of by Koul et al. and
Barza M, the microbiological analysis of aqueous samples
has not proved to be a reliable test for making therapeutic
decisions such as choosing the type of antibiotic [
In the current study, we assessed the sensitivity and
specificity of the test, using the vitreous sample as the gold
standard. Our results showed that the anterior chamber
tap have low sensitivity and specificity in all types of
endophthalmitis, except in cases of post-surgical
endophthalmitis wherein the test had a moderate to low
sensitivity (37.73%) and high specificity (93%). The high rate of
false negatives preclude its use as a screening test.
However, in the absence of the disease, the test has a 93%
chance of being negative due to its high specificity
(p < 0.04) and may be used as a confirmatory test.
Moreover, in the case of a symptomatic patient (high
prevalence), and in the absence of the gold standard (vitreous
sample), a positive anterior chamber tap might be used for
making therapeutic decisions, due to its high positive
Late onset endophthalmitis due to glaucoma procedures were not included because of the low sample size that prevented statistical analysis
Abbreviations: SPaq = aqueous sample specificity, PPVaq = positive predictive value of the aqueous sample, NPVaq = negative predictive value of the aqueous
sample, ACCaq = aqueous ample accuracy
a statistically significant data (p < 0.05)
predictive value (94%). In any other case, anterior
chamber tap results do not aid in predicting vitreous sample
results and should not substitute a vitreous tap or biopsy
for diagnosis of bacterial endophthalmitis. It is unclear
why the cases of post-surgical endophthalmitis had higher
specificity and positive predictive values than the rest.
However, the fact that all the post-surgical cases were the
consequence of anterior chamber procedures
(clearcornea phacoemulsification) may have had an impact
on the probability of false positives and true negatives.
There are several limitations that we would like to
address regarding the results and possible implications of
our study: Even with optimal conditions, vitreous sampling
(tap) have a high false negative rate. The Endophthalmitis
Vitrectomy Study (EVS) reported a positive culture rate of
only 69.3%; our study reports a similar rate (64.74%),
which probably affected the calculations of sensitivity and
specificity, thus the total amount of true positives and true
negatives may have been underestimated [
technical difficulties in collecting the sample and possible
antibiotic use, previous to the vitreous tap (71% of our cases
were sent to us from the community), may have
contributed to the low rate of positive cultures. Despite a
comprehensive medical history, not every referral had a
complete list of prior antibiotics or treatments and some
of them were sent to us in an untimely manner (days after
symptoms onset). The low vitreous positive culture rate is
a significant drawback because even the test that is
considered the Gold Standard for screening has low sensitivity
and should not be taken alone as an absolute for making
therapeutic decisions. Instead and to compensate for the
lack of sensitivity, clinicians should use the medical
history, the clinical presentation and the results of the
vitreous and aqueous tap as a whole to improve the
chances of a correct diagnosis and better inform individual
A better alternative could be to use a vitreous biopsy
instead of a vitreous tap. This will ensure enough
vitreous sample for adequate microbiological analysis
and improve the chances for a positive culture. The
culture of the tubes and aspiration cassette could also
improve the chances of isolating the causative bacteria.
However, the biopsy will possibly require a change in the
surgical setting (office to procedure room/operation
room), additional surgical equipment like drapes, cutting
and aspiration probe and scleral sutures. In a busy retina
clinic or in a clinic with limited resources (time and
space), these can lead to treatment delay and higher cost
for the patient and clinic. Another alternative is to use
molecular laboratory tests like polymerase chain reaction
and immunohistochemistry [
]. Although these tests
can potentially have better sensitivity and specificity for
bacterial endophthalmitis, regardless whether it is a
vitreous or aqueous sample; they are more expensive, need
more time for processing, as well as specialized
equipment and trained personnel [
]. Therefore, their
regular implementation as part of the endophthalmitis
workup is limited and usually restricted to large teaching
centres or research projects.
In summary, vitreous tap is a procedure used to identify
the infectious organism in cases of bacterial
]. The results from our study suggest that the
sensitivity and specificity of an aqueous tap is low and
should not be used as a substitute for a vitreous tap in
cases of suspected bacterial endophthalmitis. Only in
patients with a high suspicion of post-surgical
endophthalmitis, an aqueous tap may be used for making therapeutic
decisions in the absence of a vitreous sample. More
studies are needed to assess the value of including molecular
techniques such as polymerase chain reaction as part of
the regular workup for endophthalmitis.
ACCaq: Aqueous sample accuracy; BCVA: Best corrected visual acuity;
CF: Count fingers; FN: False negative; FP: False positive result;
logMAR: Logarithmic minimum angle of resolution equivalent; LP: Light
perception; NLP: No light perception; NPVaq: Negative predictive value of
the aqueous sample; OD: Right eye; OS: Left eye; PPVaq: Positive predictive
value of the aqueous sample; SNaq: Aqueous sample sensitivity;
SPaq: Aqueous sample specificity
There were no funds allocated to the realization of this research.
Availability of data and materials
The authors state that they have full control of all primary data and they
agree to allow Eye & Vision to review their data upon request.
CSJ: review of medical records, recollection of data and writing of manuscript.
VSV: review of medical records, recollection of data and writing of manuscript.
GSV: review of the results and writing of manuscript. ASP: Results analysis
and writing of manuscript. JGN: Writing of manuscript and approved final
manuscript. JFG: Statistical analysis, writing of manuscript and approved
final manuscript. GGA: Writing of the manuscript, recollection of data and
approved final manuscript. VMC: Writing of manuscript and approved final
manuscript. RVM: Original Idea, review and collection of data. Statistical
analysis, major writing of the manuscript and approved final manuscript.
All authors read and approved the final manuscript.
Ethics approval and consent to participate
The study was approved by the hospital’s Internal Review Board. Due to the
study’s retrospective nature, no informed consent was needed.
Consent for publication
The authors declare that they have no competing interests.
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