Criteria abstracted from The Users' Guide to Medical Literature, from the Health Information Research Unit and Clinical Epidemiology and Biostatistics, McMaster University

Highlighted lines and questions below provide links to the pertinent description of criteria in The EBM User's Guide, now available at the Canadian Centres for Health Evidence

Specific questions in this review are based on Jaeschke RZ, Meade MO, Guyatt GH, Keenan SP, Cook DJ. How to use diagnostic test articles in the intensive care unit: diagnosing weanability using f/Vt. Crit Care Med. 1997;25(9):1514-2. [abstract]

Article Reviewed:

Please visit the new Evidence Based Journal Club Reviews

Blind and bronchoscopic sampling methods in suspected ventilator-associated pneumonia: A multicentre prospective study.

Mentec H, May-Michelangeli L, Rabbat A, Varon E, Le Turdu F, Bleichner G.

Intensive Care Med. 2004; 30-1319-1326. [abstract]

Reviewed by Mark Weber RN CPNP, West Virginia University Children's Hospital, Morgantown West Virginia

Review posted September 5, 2005

I. What is being studied?

Study objective:

To compare four sampling methods: blind tracheal aspirate (blind TA), blind protected telescoping catheter (blind PTC), bronchoscopic PTC and bronchoscopic bronchoalveolar lavage (bronchoscopic BAL) in the diagnosis of ventilator-associated pneumonia (VAP).

Study design:

A prospective multicenter study.

The patients studied:

Adult patients ages 50-74 from five ICUs who were receiving mechanical ventilation for more than 48 hours and had clinically suspected VAP. All patients also had no change in antibiotic regimen within 72 hours of admission to study. The criteria for suspected VAP were: (1) new and persistent alveolar infiltrates on chest radiograph, (2) purulent tracheal aspirates or at least two of the following: fever of 38°C or more or hypothermia less than 36.5°C, leukocytosis more than 10x10¹²/l or leukopenia less than 4x10¹²/l, decrease in PaO2/FIO2 ratio of more than 20 mmHg.

II. Are the results of the study valid?

Primary questions:

1. Was there an independent, blind comparison with a reference standard?

Yes. Physicians in charge of the patients used the 1992 International Consensus Conference criteria for VAP (1) as a reference standard. This diagnostic criteria, however, bases mainly on bronchoscopic BAL A threshold of ≥ 10⁴ cfu/ml was set for a positive culture. It is the best available reference standard. Performing a lung biopsy culture on each patient is not a necessary or feasible reference standard. All patients underwent the four sampling methods in the same order during the same procedure. It was not mentioned who performed the specimen samplings, and whether they had prior knowledge on patients' clinical condition. This information may have influence the quality of sampling procedures.

The article did not mention if the specimens sources were blinded to the laboratory personnel. It simply states that all four cultures were sent to the lab within 15 minutes for direct examination and quantitative culture. We know that the lab worker had to know the PTC culture due to the fact that the catheter tip was sent with the culture.

2. Did the patient sample include an appropriate spectrum of patients to whom the diagnostic test will be applied in clinical practice?

Yes. Patients were drawn from five ICUs. A well-defined clinical criteria for suspicion of VAP was used. This criteria would likely include a wide spectrum of the VAP in clinical practice. The length of mechanical ventilation before inclusion ranging 5-15 days supports this assumption. The study included 63 patients with ages ranging from 50-74 years. There were 34 males and 29 females. The general patient types comprised of 83% medical patients, 16% unscheduled surgical patients and 2% scheduled surgical patients.

There were a high number of medical patients in the study population. This may not appropriately represent a typical population of a PICU. In addition to the obvious - this study did not include children - many PICU's are mixed units caring for medical and surgical patients, including trauma and post-op cardiac surgery patients. The study did not seem to include this diverse population.

Secondary questions:

3. Did the results of the test being evaluated influence the decision to perform the reference standard?

No, each patient enrolled in the study underwent all 4 types of sampling methods in the same order. The order for culturing was blind TA, blind PTC, bronchoscopic PTC and bronchoscopic BAL.

4. Were the methods for performing the test described in sufficient detail to permit replication?

Yes

Yes. The authors provide electronic supplementary material that includes the sampling protocol used. (available at: http://dx.doi.org/10.1007/s00134-004-2284-7)

III. What are the results?

1. Are the test's sensitivity, specificity, and likelihood ratios presented (or are the data necessary for their calculation provided)?

The researchers do not provide likelihood ratios for the results, although there is sufficient data available for their calculation.

Method	Threshold	Sensitivity	Specificity	+LR	-LR	95% CI	auROCc
Blind TA	≥ 105	82	67	2.48	0.27	1.26-4.83	0.783
Blind PTC	≥ 103	62	94	10.3	0.40	1.45-3.35	0.829
Blind PTC vse	≥ 103	82	90	8.20	0.20	1.98-10.24	0.902
Broncho PTC	≥ 103	71	94	11.83	0.31	1.57-4.26	0.848
Broncho PTC vse	≥ 103	81	100	inf	0.19	2.05-8.59	0.913
Broncho BAL	≥104	94	100	inf	0.06	8.64-11.57	0.977

Vse= visible secretions expelled from catheter

A goal LR of > 10 for a +LR and < 0.1 for a -LR are generally accepted as highly useful values (2). The blind PTC did come close to meeting the criteria of useful +LR and -LR . To positively diagnose VAP the Blind PTC would be adequate, but to rule the disease state out the presence of vse would be desirable.

The blind PTC provided quite high LRs (i.e. 10.3 for positive test and 0.4 for negative test). Using the likelihood ratio nomogram (3), a positive culture from blind PTC would raise pre-test probability of 65% to post-test probability of 95%. This finding helps confirm VAP in a population with high pre-test probability. On the other hand, negative culture from blind PTC would shift probability of having VAP from 65% (pre-test) down to 42% (post-test).This post-test probability may not be low enough for a clinician to discontinue antibiotics.

The blind PTC was equally accurate in compared with broncho PTC. The visualization of secretions expelled from the catheter enhanced the test performance in excluding the disease (-LR of 0.2). Again, using likelihood ratio nomogram, the negative culture from the blind PTC with visible secretion would decrease probability of having VAP from 65% (pre-test) down to only 27% (post-test) which could be low enough for a clinician to discontinue antibiotics.

The authors also used areas under the receiver operating characteristics curve (auROCc) to determine the best threshold and the operating characteristics of the four sampling techniques. The auROCcs of blind and bronchoscopic PTC did not differ significantly, either in the 52 analyzed patients (p=0.77) or in the subgroup with visible secretions expelled from the catheter for both PTC samplings (p=0.60). When samples with visible secretions were considered, blind PTC had much better operating characteristics that the auROCc for blind PTC was almost equal to that of brochoscopic BAL (0.902 vs. 0.977, p=0.22). Hence, the blind PTC with visible secretions is a great alternative sampling technique that provides almost equivalent diagnostic performance for VAP in compared with a reference standard, brochoscopic BAL.

IV. Will the results help me in caring for my patients?

1. Will the reproducibility of the test result and its interpretation be satifactory in my setting?

Limited. In the larger teenage patients the use of the PTC can be applicable in the diagnosis of VAP. It should be noted that the reproducibility of this study is limited by the smaller patient size seen in the PICU. The relatively large size of the PTC catheter would limit it from being used through smaller endotracheal tubes.

2. Are the results applicable to my patients?

Perhaps. Although the patient population in the study was only adults, the blind PTC might provide the same diagnostic performance in pediatric population. The blind PTC is more practical compared with brochoscopic BAL in small children. However, without formal study, applicability to pediatrics is speculative.

3. Will the results change my management?

Yes. The +LR of PTC was much higher than that of Blind TA (10.3 vs. 2.48), which has been the present standard of respiratory tract culturing in our PICU. The blind PTC is very helpful in confirming VAP diagnosis but not as good in ruling out VAP if no secretion expelled from the catheter. When the pre-test probability is very low (i.e., fever and changes in secretion without pulmonary infiltrate), blind tracheal aspirate may be adequate to rule out VAP (-LR of 0.27).

4. Will patients be better off as a result of the test?

Yes. Compared to the present standard of culturing being Blind TA, PTC provides a better +LR as noted above. The long term benefits to the patient would come in the tailoring of the antibiotic regimen as coverage is narrowed down or stopped based on culture results.

References:

1. Pingleton SK, Fagon JY, Leeper KV. Patient selection for clinical investigation of ventilator-associated pneumonia. Criteria for evaluating diagnostic techniques. Chest 1992;102:553S-556S. [citation]
2. Jaeschke R, Guyatt GH, Sackett DL. Related Articles, Links Users' guides to the medical literature. III. How to use an article about a diagnostic test. B. What are the results and will they help me in caring for my patients? The Evidence-Based Medicine Working Group. JAMA. 1994 Mar 2;271(9):703-7. [full-text]
3. Fagan TJ. Letter: Nomogram for Bayes theorem. N Engl J Med 1975; 293. 257.

 

Comments

Back to the EB Journal Club Index