Comparative Evaluation of the Role of Nonbronchoscopic and Bronchoscopic Techniques of Distal Airway Sampling for the Diagnosis of Ventilator-Associated Pneumonia.

BACKGROUND: The diagnosis of ventilator-associated pneumonia (VAP) remains a challenge, with clinicians mainly relying on clinical, radiological, and bacteriologic strategies to manage patients with VAP. AIMS: To compare the results of non-bronchoscopic and bronchoscopic techniques of distal airway sampling for the diagnosis of VAP. SETTINGS AND
DESIGN: This was a single-center prospective diagnostic accuracy study done in the 14-bedded intensive care unit of a tertiary care referral hospital.
MATERIALS AND METHODS: Patients aged ≥18 years, on mechanical ventilation for ≥48 h, and with clinical suspicion of VAP (fever, leukocytosis, and increased tracheal secretions) either on admission or during their stay were included. Every patient underwent both procedures for sample collection, first non-bronchoscopic protected bronchoalveolar lavage (NP-BAL) and then bronchoscopic BAL (B-BAL). Clinical Pulmonary Infection Score (CPIS) was calculated for each patient and the collected samples were evaluated in laboratory using standard microbiological techniques. STATISTICAL ANALYSIS USED: The sensitivity, specificity, positive predictive value, and negative predictive value of NP-BAL and B-BAL for the diagnosis of VAP were calculated taking CPIS score of >6 as index test for the diagnosis of VAP.
RESULTS: Sixty patients were included in the study. Both NP-BAL and B-BAL had concordance with the CPIS at 69.1%. The concordance between NP-BAL and B-BAL was better at 67.6% with a kappa coefficient of 0.064 (P = -0.593). The yield and sensitivity of NP-BAL were comparable to that of B-BAL.
CONCLUSIONS: The blind NP-BAL is an equally effective method of airway sampling and could be a better alternative to replace more invasive B-BAL for microbiologic diagnosis of VAP. Copyright:

INTRODUCTION

Ventilator-associated pneumonia (VAP) has an incidence of 5%–40% in critically ill patients and up to 27% in mechanically ventilated patients.[1] Mortality rates in such patients range from 20% to 50% and may reach 70% when caused by multidrug resistant and invasive pathogens.[123] Poor outcome could be due to a delay in the initiation of appropriate therapy because of late diagnosis of VAP and also due to unnecessary treatment because of incorrect diagnosis.

The diagnosis of VAP remains a challenge and has conventionally been made based on clinical signs or microbiologic diagnostic techniques. The clinical signs and symptoms lack both sensitivity and specificity and the standard microbiologic diagnostic procedure is still an open-ended debate.[4] The Clinical Pulmonary Infection Score (CPIS) was proposed by Pugin et al.[5] It is based on six variables (fever, leukocytosis, tracheal aspirates, oxygenation, radiographic infiltrates, and semiquantitative cultures of tracheal aspirates). As a diagnostic tool for VAP, CPIS value of >6 has sensitivity and specificity of 93% and 100%, respectively.[5]

Microbiological diagnostic techniques include invasive or non-invasive sampling from either proximal airway or distal airway. This study was designed with the aim to compare the results of non-bronchoscopic and bronchoscopic techniques of distal airway sampling for the diagnosis of VAP. We determined the diagnostic accuracy of two quantitative distal airway sampling techniques, non-bronchoscopic protected bronchoalveolar lavage (NP-BAL) and bronchoscopic BAL (B-BAL), with the objective to assess their role in the diagnosis of VAP.

MATERIALS AND METHODS

Study design and participants

We conducted a prospective diagnostic accuracy study over a period of 12 months in patients with clinical suspicion of VAP. We compared two methods of distal airway secretions to diagnose VAP, NP-BAL and B-BAL. The study was performed at the 14-bedded intensive care unit (ICU) of a tertiary care referral hospital after approval from the institutional human research ethical committee (IEC No. 54/15). Informed consent from the patient's next of kin was taken regarding enrollment of the patient in the study and that they could withdraw at any point from the study without impact on treatment.

Inclusion and exclusion criteria

Eligible participants were patients getting admitted in ICU, ≥18 years of age, of any ethnic group, on mechanical ventilation for ≥48 h, and with clinical suspicion of VAP, either on admission or during their stay. Patients with any contraindication to bronchoscopy like bleeding diathesis, profound refractory hypoxemia, and malignant cardiac arrhythmias were excluded from the study.

Clinical suspicion of pneumonia was based on the criteria given by Johanson et al.[6] suspicion of pneumonia with an infiltrate on the chest radiograph and presence of two of the following- leukocytosis >12 × 109 ml−1, fever >38.3°C, or purulent tracheobronchial secretions.

Data collection

All the data collection was done by the principal investigator, while NP-BAL and B-BAL sample were collected by the senior resident posted in ICU. Demographic information, symptoms, symptoms duration, and nasopharyngeal temperature of patient were recorded at enrollment. Blood sample was taken daily for total leukocyte count and number of band forms; chest radiography was performed to identify new infiltrate and arterial blood sample to assess partial pressure of oxygen/fraction of inhaled oxygen (PaO2/FiO2). Tracheal secretions were assessed every day for character (purulent or not) when doing tracheal secretion suctioning using closed suction system.

Study procedures and specimen collection

In each patient, having suspicion of VAP based on clinical criteria by Johanson et al.,[6] CPIS[5] was calculated on the day of sample collection. Two respiratory samples were collected from all patients, which included NP-protected BAL (NP-BAL or Mini-BAL) and B-BAL. To avoid contamination of the distal airways, the NP-BAL sampling was performed first followed by B-BAL. B-BAL and NP-BAL sampling was done by the senior resident doctor. Before either procedure, FiO2 was adjusted to 1.0 for 30 min. All the vital signs including heart rate, blood pressure, temperature, and oxygen saturation were monitored during the entire procedure. For the ease of catheter and flexible bronchoscope insertion without the need to interrupt the mechanical ventilation, a special elbow adaptor mounted on the endotracheal tube was used. The seal on the hole of elbow adaptor was opened during the procedure and therefore airway pressure was not maintained during the procedure. 3–5 mg of intravenous midazolam was used for sedation if required.

Nonbronchoscopic-protected bronchoalveolar lavage (or mini-bronchoalveolar lavage)

NP-BAL was performed by double catheter technique. A sterile suction catheter of size 16 Fr was cut 2–3 cm from the distal end to give a final length of about 47–48 cm. It was inserted through the endotracheal tube and blindly advanced into the distal airways till resistance was felt, and then, a second 50 cm long, sterile suction catheter of 8 Fr size was passed through the first catheter and advanced as far as possible. 20 mL of sterile saline was then instilled into the distal airways through the inner tube, which was aspirated and collected in a sterile container. Quantity of the aspirate was recorded. If the aspirated fluid was less than 5 mL, the procedure was repeated.

Bronchoscopic bronchoalveolar lavage

Oral airway was used to prevent biting and damage to scope. The bronchoscope (Pentax Medical, Orangeburg, New York, USA) was introduced and the tip was wedged to distal bronchi draining the bronchopulmonary segment of interest as determined by chest radiograph. Right lower lobe was sampled in case of diffuse/bilateral lung infiltrates. Then, 20 ml of sterile saline was instilled through the working lumen of bronchoscope and gently aspirated by suction. Quantity of aspirate was noted.

After bronchoscopy, FiO2 was kept at 1.0 for 1 h. For cleaning of bronchoscope, after every procedure, the external surface of fiberoptic bronchoscope was cleaned by immersing it in detergent solution. Suction port and instrument channels were cleaned with cleaning brush and by aspirating the detergent. After rinsing with water, the scope was disinfected by immersing it in 2% glutaraldehyde for 30 min. Thereafter, distal end of scope was immersed in 70% alcohol followed by aspiration for 5 s. Air was suctioned to dry the inside of the suction port. The surface was cleaned and dried with alcohol moistened cloth.

Laboratory tests

The collected samples were immediately transported for bacteriologic examination and quantitative cultures to our microbiology laboratory within 1 h of collection, where further processing was done. NP-BAL and B-BAL samples were divided into two: the first half was centrifuged (1500 rpm.min−1 for 10 min) and used for Gram stain. Semiquantitative culture of the second half of the samples was done using calibrated loop method,[7] in which 0.01 ml of specimen was plated directly into chocolate agar, blood agar, and MacConkey agar. Plates were incubated at 35°C–37°C for 24 h. Numbers of colonies grown were converted into number of CFU.ml−1 as follows:

1 bacterial colony grown corresponded to 102 CFU.ml−1

10 bacterial colonies grown corresponded to 103 CFU.ml−1

100 bacterial colonies grown corresponded to 104 CFU.ml−1

1000 bacterial colonies grown corresponded to 105 CFU.ml−1.

Bacterial identification was done using standard microbiologic techniques. The threshold of 104 CFU/ml was applied to NP-BAL and B-BAL for the diagnosis of VAP.

Statistical analysis

The results were collected, tabulated, and statistically analyzed by an Statistical Package for the Social Sciences version 16.0 (IBM Corporation, Chicago, Illinois, USA). Results were expressed in frequency, percentage, or mean and standard deviation when appropriate. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of NP-BAL and B-BAL for the diagnosis of VAP were calculated by defining true or false positives and true or false negatives against the reference standard of CPIS score of >6. The compatibility of B-BAL and NP-BAL results for all cases was evaluated with kappa statistics. A two-tailed P < 0.05 was considered statistically significant.

Over a period of 8 months, 189 patients getting admitted to our ICU were assessed and based on clinical criteria by Johanson et al.[6] 68 patients had suspicion of VAP. Among those 68 patients, 8 patients were excluded and 60 patients with high clinical suspicion of VAP were prospectively evaluated [Figure 1]. A definite diagnosis of VAP (CPIS ≥ 6) was present in 31 patients. There was no dropout during the study period.

Figure 1

Trial algorithm. *Ventilator-associated pneumonia, †Clinical Pulmonary Infection Score, ‡Nonbronchoscopic-protected bronchoalveolar lavage, §Bronchoscopic bronchoalveolar lavage

RESULTS

Baseline characteristics of patients were comparable and are described in Table 1. The most common primary diagnosis [Table 2] at admission was cerebrovascular accident followed by postoperative cases, metabolic encephalopathy, acute inflammatory demyelinating polyneuropathy, acute exacerbation of chronic obstructive pulmonary disease, renal failure, and others. The most common indication of mechanical ventilation was impending respiratory failure followed by low Glasgow Coma Scale.

Table 1

Baseline characteristics of patients at the time of sampling

Variables	Results, mean±SD (range)
Age (years)	55.27±16.88 (18-85)
Male: female ratio	13:7
Duration of ICU stay (days)	10.21±5.67 (3-31)
Duration of mechanical ventilation (days)	9.82±5.66 (3-30)
Duration of increased tracheal secretions (days)	4.26±1.33 (2-8)
Duration of fever (days)	5.02±2.45 (0-14)
Total leukocyte counts (mm−3)	17,790±10,910 (1040-41,610)
pO2 (mmHg)	121.10±33.05 (25.58-205)
pCO2 (mmHg)	32.89±13.22 (4.9-61.24)
CPIS score	5.88±1.90 (3-11)
Modified APACHE II	20.30±7.06 (5-34)

ICU=Intensive care unit, pO2=Partial pressure of oxygen, pCO2=Partial pressure of carbon-di-oxide, CPIS=Clinical pulmonary infection score, APACHE=Acute physiology and chronic health evaluation, SD=Standard deviation

Table 2

Distribution of provisional primary disease of patients

Diagnosis	Number of cases (%)
Cerebrovascular accident	12 (20)
Postoperative cases	10 (16.6)
Metabolic encephalopathy	7 (11.6)
Acute inflammatory demyelinating polyneuropathy	6 (10)
Acute exacerbation of COPD	6 (10)
Renal failure	6 (10)
Road traffic accident	5 (8.3)
Acute meningoencephalitis	5 (8.3)
Septic shock	3 (5)

COPD=Chronic obstructive airway disease

There was no significant complication observed during or after sampling procedures. Samples of NP-BAL and B-BAL were collected from all 60 patients. The diagnostic value of both sampling techniques is described in Table 3. B-BAL had the higher yield and sensitivity. The results of NP-BAL were comparable to that of B-BAL. The concordance between NP-BAL and B-BAL was better at 67.6% with a kappa coefficient of 0.064 (P = −0.593). The concordance and kappa coefficient of both sampling techniques with CPIS is shown in Table 4. NP-BAL had a concordance with CPIS at 69.1% and a kappa value of 0.344 (P = −0.002). B-BAL had concordance of 69.1% and a kappa value of 0.334 (P = −0.001).

Table 3

Diagnostic value of various sampling techniques

Sampling techniques	Yield (%)	Sensitivity (95% CI)	Specificity (95% CI)	PPV (95% CI)	NPV (95% CI)
NP-BAL	75.0	89.4 (74.2-96.5)	43.3 (25.9-62.3)	66.6 (51.9-78.8)	76.4 (49.7-92.1)
B-BAL	80.8	94.7 (80.9-99.0)	36.6 (20.5-56.1)	65.5 (51.3-77.4)	84.6 (53.6-97.2)

CI=Confidence interval, PPV=Positive predictive value, NPV=Negative predictive value, NP-BAL=Nonbronchoscopic protected bronchoalveolar lavage, B-BAL=Bronchoscopic bronchoalveolar lavage

Table 4

Concordance of various sampling techniques with clinical pulmonary infection score

	κ	Concordance (%)	P
CPIS versus NP-BAL	0.344	69.1	0.002
CPIS versus B-BAL	0.334	69.1	0.001
NPBAL versus B-BAL	0.064	67.6	0.593

NP-BAL=Nonbronchoscopic protected bronchoalveolar lavage, B-BAL=Bronchoscopic bronchoalveolar lavage, CPIS=Clinical pulmonary infection score

Appendix Tables 1 and 2 show the organisms isolated in NP-BAL and B-BAL, respectively. The most common organism isolated in NP-BAL was Pseudomonas aeruginosa (35%), followed by Acinetobacter baumanii (15%), Escherichia coli (15%), Klebsiella Pneumonia (10%), Staphylococcus aureus (6.6%), Enterobacter cloacae (6%), and Proteus mirabilis (5%).

Appendix Table 1

Organisms isolated in nonbronchoscopic-protected bronchoalveolar lavage

Organisms	Number of samples (%)
Acinetobacter baumanii	9 (15)
Acinetobacter lowfeii	0
Enterobacter cloacae	4 (6)
Escherichia coli	9 (15)
Klebsiella pneumonia	6 (10)
Proteus mirabilis	3 (5)
Pseudomonas aeruginosa	21 (35)
Staphylococcus aureus	4 (6.6)

Appendix Table 2

Organisms isolated in bronchoscopic bronchoalveolar lavage

Organisms	Number of samples (%)
Acinetobacter baumanii	14 (23.3)
Acinetobacter lowfeii	2 (3)
Enterobacter cloacae	5 (8.3)
Escherichia coli	10 (16.6)
Klebsiella pneumonia	4 (6.6)
Proteus mirabilis	2 (3.3)
Pseudomonas aeruginosa	20 (33.3)
Staphylococcus aureus	4 (6.6)

In B-BAL also, the most common organism isolated was P. aeruginosa (33.3%), followed by A. baumanii (23.3%), E. coli (16.6%), E. cloacae (8.3%), Klebsiella pneumonia (6.6%), S. aureus (6.6%), P. mirabilis (3.3%), and Acinetobacter lowfeii (3%).

Microbial cultures were positive in 48 samples of NP-BAL and 55 samples of B-BAL. Perfect qualitative concordance (organism and antibiotic sensitivity) among NP-BAL and B-BAL was seen in 44 out of 60 cases.

DISCUSSION

At present, there is no universally accepted gold standard criterion for the diagnosis of VAP and though clinicians mainly rely on clinical, radiological, and bacteriologic strategies to manage patients with VAP,[1] up to two-third of patients treated for VAP may not actually have VAP.[8] Proximal airway specimen collected by endotracheal aspirate has conventionally been the microbiologic method used to diagnose VAP. Tracheal secretions which are although easily and non-invasively obtainable, but very often non-conclusive results are obtained by microscopic evaluation and culture of these samples,[9] because the upper respiratory tract of most ICU patients is colonized with potential pulmonary pathogens, even if deep pulmonary infection is not present. Thus, distal airway sampling techniques emerged, which include bronchoscopic as well as NP ways.

Numerous studies have postulated that invasive diagnostic methods like quantitative cultures of distal airway specimens obtained by using NP-BAL and B-BAL have high diagnostic yields.[71011] However, bronchoscopy, which itself is a high risk procedure in patients of pneumonia presenting with thrombocytopenia and hypoxemia, requires experienced operators and also, increases the cost of care.

In an attempt to overcome these limitations, NP distal airway sampling method called mini BAL (mini-BAL) or NP-protected BAL (NP-BAL) came out. NP-BAL is non-invasive and done blindly, using an endobronchial catheter that is wedged in the tracheobronchial tree.[1213] Although simple and inexpensive, diagnostic accuracy of this blind sampling method has not been proven over bronchoscopic sampling.

Our study evaluated the role of two distal airway sampling techniques, NP-BAL and B-BAL for the microbiologic diagnosis of VAP. The American Thoracic Society (ATS) and the Infectious Diseases Society of America (IDSA) have recommended fetching of samples for culture and microbiology from lower respiratory tract.[1]

The two distal airway sampling techniques, NP-BAL and B-BAL, that are assessed in our study showed a fair level of agreement with CPIS (NP-BAL k = 0.344, P = 0.002 and B-BAL k = 0.344, P = 0.001). Bronchoscopic techniques for respiratory sample collection has been shown by ATS/IDSA guidelines to reduce 14-day mortality when compared with a clinical strategy (16.2% vs. 25.8%, P = 0.02)[1] as well as significantly more antibiotic-free days (11.5 ± 9.0 vs. 7.5 ± 7.6, P < 0.001) compared to guideline-based clinical diagnosis alone.[14]

The procedure of specimen collection by B-BAL (when compared to NP-BAL) requires high expertise and special equipment, is more invasive, more time-taking (thus more compromise with oxygenation and ventilation of patient), more chances of contamination of samples through the bronchoscopic channel and more cost.[15] NP-BAL which is a simple procedure that can be performed by resident doctors and paramedics (nurses) posted in the ICU after a small demonstration and with a sensitivity and specificity of 89.4% and 43.3% could prove to be inherently advantageous and cheaper alternative over B-BAL, especially in developing countries, where bronchoscopy guided specimen collection facility may not be available in every ICU. In our study, the concordance observed between NP-BAL and B-BAL was 67.6% (k = 0.064, P = 0.593). The high concordance seen (even though NP-BAL is a blind procedure) proves the fact that NP-BAL adequately represents the distal airway secretions and could efficiently diagnose VAP. A large multi-centric study by the Canadian Critical Care group observed no difference in clinical outcome among patients treated for VAP based on bronchoscopic or NP-BAL procedures.[16] Thus stating similar efficacy between B-BAL and NP-BAL for diagnosis of VAP. In an another study by Rouby et al., they used mini-BAL for the diagnosis of nosocomial pneumonia and concluded that mini-BAL is an easily applicable, repeatable, inexpensive, and highly efficacious alternative diagnostic tool over bronchoscopic methods of respiratory sampling.[17]

The criteria with high sensitivity should guide the diagnosis of VAP. In our study we found sensitivity of NP-BAL to be 89.4% and of B-BAL to be 94.7%. This perspective is based on the point that not treating a patient with pneumonia in all likelihood increases the risk to life far greater than that of unnecessary antibiotic administration.[1] In an autopsy study by Rouby et al., the sensitivity and specificity of NP-BAL was 70 and 69% respectively, using post-mortem histologic and bacteriologic analysis of lung as the gold standard for the diagnosis of VAP.[18]

Pugin et al. used CPIS as the diagnostic criteria for VAP and found that sensitivity, specificity, and PPV of NP-BAL were 73%, 96%, and 92%, respectively.[5] Many other studies have also shown that NP-BAL has high sensitivity (70%–70%) and specificity (69%–69%) depending on the variable criteria used to diagnose VAP.[5171920]

We demonstrated a yield of 75% with NP-BAL and 80.8% with B-BAL. The most common organism isolated in both NP-BAL and B-BAL was P. aeruginosa followed by A. baumanii and E. coli. Perfect qualitative concordance (organism and antibiotic sensitivity) between NP-BAL and B-BAL was seen in 44 out of 60 cases (73.33%), demonstrating a good agreement for the type of microorganisms. In a study by Kollef et al., NP-BAL done by a respiratory physiotherapist has shown good microbiologic agreement (83.3%) with bronchoscopic protected brush.[20] These results suggests that blind sampling technique like NP-BAL is a good modality for microbiologic diagnosis of VAP.

Strengths and limitations

Firstly, we used CPIS as the reference standard for our study. Though CPIS has high sensitivity for the diagnosis of VAP,[5] there are studies that question the usefulness of CPIS for the diagnosis of VAP.[2122] Thus, in the absence of the gold standard for the diagnosis of VAP, the validity of the exact operating characteristics (sensitivity, specificity, PPV, and NPV) for both the sampling techniques may be questioned. To determine the precise diagnostic yield of bronchoscopic and NP procedures, lung tissue examination at autopsy (both bacteriological and histological examination) has been used as a gold standard.[182324] But, this has a limitation of not being useful in clinical decision making. Secondly, as B-BAL sample was obtained after NP-BAL, there could be a concern of alveolar fluid being diluted for the B-BAL sample. However, most of the previous studies have followed this protocol and did not find any significant effect of this dilution on microbiology of the sample.[719202526] To nullify this further we did not repeat the sterile saline instillation during NP-BAL and thus collected the specimen in one go in majority of the cases.

CONCLUSIONS

The strong correlation between two methods demonstrates that NP-BAL is an equally efficient sampling method with a high sensitivity and could be a better alternative to replace B-BAL for microbiologic diagnosis of VAP, especially in resource-limited settings. However, we suggest that the data from our study should be validated in a prospective manner in future studies done at multiple center with greater numbers of cases to provide a good-quality evidence for the same.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.