A study to evaluate two sampling methods in ...

A study to evaluate two sampling methods in determining surgical theatre air quality

Tshokey

Submitted for the degree of MD in Medical Microbiology

Postgraduate Institute of Medicine (PGIM) University of Colombo

2010

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Statement of Originality

I hereby state that the work presented here in this thesis is my own, and that no part of this has been submitted earlier or concurrently for any other degree.

Dr. Tshokey MBBS, Dip.Med.Micro (Col) MD Trainee (2008-2010) MD in Medical Microbiology

Verified by Supervisor:

Dr.(Mrs) Pranitha Somaratne MBBS, Dip.Med.Micro, MD Consultant Microbiologist Medical Research Institute Colombo

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Dedication This dissertation is dedicated with love to my family who sacrificed in different ways While I dedicated my life to the study of ‘Microbes and Infections’ away from home for too long. My Father and Late Mother who were deprived of a son’s love and care when they needed it most at their old age. My wife who sacrifices so much everyday for the family without me but never complained. For my two children who so far have not experienced the real love of a father at home.

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List of abbreviations

bcp – Bacteria carrying particles BHT – Bed Head Ticket CDC – Centre for Disease prevention and Control, Atlanta CFU – colony forming units CFU/m3 – CFU per cubic meter CV – Conventional Ventilation theatre HAIs – Healthcare-Associated Infections HEPA - high-efficiency particulate air HTM - Health Technical Memorandum LAF – Laminar air flow theatre NHSL – National Hospital of Sri Lanka OT – Operating theatres OT-O – Orthopedic Operating theatre SSIs - Surgical Site Infections UCV – ultra-clean ventilation

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Abstract Background: Healthcare-associated infections (HAIs) are an important cause of morbidity and mortality, prolongs hospital stay, increases antibiotic usage, add costs. Surgical Site Infections (SSIs) are a very important topic of concern in the ever changing times of health care. There are countries and hospitals that have just begun to realize the importance of preventing and surveillance of SSIs while there are others who have reached heights in the implementation of protocols of Infection controls for HAIs in general and SSIs in particular, depending on the types of services the hospital renders.

Of the many inseparable contributing factors in SSIs, the air contamination in the Operating Theatre (OT) is an important factor that has called for the requirements of different air supply systems, cleaning protocols, use of surgical protective suites and limiting of individuals in the OTs. The use of antibiotic prophylaxis has contributed tremendously to the prevention of SSIs especially in the major surgeries and surgeries conducted in ordinary air supplies. It is an important task for the clinical microbiology laboratory to monitor the quality of air in the surgical theatre. But controversy exists over the extent and frequency of microbiological surveillance of operation theatres. Evaluation of the quality of the air in the theatres can be performed routinely by microbiological sampling and particle counting as part of regular infection control programs. These are especially required in the commissioning of the new operation theatres, investigation of clusters or outbreaks of infections or to validate a change in the ventilation system of the theatre.

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Objectives: The objectives of the project were as follows: 1. To compare two methods of air sampling, the traditional settle plate method and a commercial slit-air-sampler 2. To find the difference of air contamination in the theatre with laminar air flow (LAF) and the one with conventional (CV) ventilation. 3. To compare the air contamination in an empty OT before starting the list in the morning and towards the end of the morning list (during use), both in the LAF and CV theatres. 4. To determine any differences in contamination levels at different areas within an OT. 5. To determine the common bacterial species isolated from the theatre air. 6. To determine any surgical site infections in the operated patients till the time of discharge from hospital.

Study design and setting: This is a prospective study. The samples were collected from two OTs of Orthopedic unit of NHSL, one with LAF and the other with CV ventilation system. Sampling was done in the morning before start of the morning list (empty OT) and towards the end of the morning list. Sampling was done 8 wks, two days a week. (Period sampling)

Methods and materials: Two methods of air sampling were used, the conventional settle plate method and a slitsampler. Settle plate method Settle plate method was directly adopted from the method described in the Microbiology text book, Mackie and McCartney, Practical Medical Microbiology, 14th edition (1999), 15; 908, and reproduced below. The calculation of CFU/m3 was adapted from M T parker (1978).

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a) Blood agar was poured and dried of any surface moisture. All plates were marked for identification. b) In the theatre the plates were uncovered in their chosen places and exposed by keeping 1 meter above the floor, 1 meter away from the side walls for 10 minutes, then at once the lids replaced. c) The plates were then incubated aerobically at 37oC overnight. d) The colonies were counted, using hand lens when necessary. The colonies were identified using basic tests. e) The number of colony forming units (CFU) per square meter per minute (CFU/m2/min) is calculated to give the settling rate. Here the 90mm disposable plastic plates were used which gives a plate surface area of 63.6cm2. From the number of CFU in a 90mm plate after 10 minutes exposure, the contamination level was calculated by using the formula of MT Parker (1978) to get the values of CFU/m3.

Slit Sampler The slit sampler was used as per the descriptions in the ‘User’s Manual’ for ‘HiAirflow 90’ of the product from HiMedia Laboratories, India described below. a) The device was charged and cleaned with a disinfectant (70% alcohol). The head can also be sterilized, but sterilization was not done during my use. b) From the control panel the air flow rate and the total volume to be sampled were selected. Here the flow rate chosen was 100L/min & the total volume sampled was 1000L or 1 m 3, therefore the sampling time is 10 minutes. c) A 90 mm blood agar plate was placed into the head of the device and covered with the perforated cap. d) The air was drawn through the perforated head at the set rate of sampling for 10 mins. The inflow air impacts on the surface of the medium and deposits the particles it carries. e) The plates were incubated under similar conditions as for settle plate and the colonies counted and identified. 7

f) The most probable number of microorganisms in the volume of air sampled was calculated from the ‘Conversion Table HiAirflow 90’ given in the manual. (Appendix). This gives the number of CFU/m3.

Results: The two methods of sampling show a distribution pattern in scatter plot, in a way that almost all high or low values in the sampler correlates with the Settle plate values. The two methods showed moderate correlation by Pearson’s correlation test but the correlation was highly significant. This indicates that, in the unavailability of a sampler the settle plate method may be a fair substitute. The OT with CV was found to be more contaminated when empty as well as during use, in both methods as expected. The differences in contamination between the CV and LAF theatre was found to be significant by the ‘t’ test, by both the methods while empty and during use. Overall, the significance of the difference in contamination was higher when the sampling was done by the commercial sampler. This may mean that the sampler is more sensitive than the settle plate method in detecting the difference. Difference in contamination before and after surgery is significant in both the theatres, but significance is less in LAF (P = 0.027 vs 0.7). But this moderate correlation is highly significant at p value < 0.001 level (2-tailed). This graph and r value shows highly significant moderate correlation between two methods with substantial relationship according to Guilfords guidelines. This simply indicates that where the sampler is not available, the settle plate can be an acceptable substitute sampling method, since it provides moderate correlation but highly significant results with the gold standard (the sampler). Further to this observation, the comparison was done for the calculated CFU/m3, the actual contamination per volume of air. Table 3: Correlation between settle plate and sampler for CFU/m3 No. of CFU/m3 – Settle Plate- Sampler Pearson Correlation

.592

Sig. (2-tailed)

.000

N

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Here too the correlation was moderate (r = 0.592) but the correlation is highly significant with p value < 0.001 level (2-tailed).

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Since the correlation was high, sub analysis was done to see the correlation in different cut off points; theaters and time of measurements, but it did not show significant differences. 2. To find the difference of air contamination in the theatre with laminar air flow and the one with conventional ventilation. Test statistics were applied to identify differences between theaters before and during the surgeries. Two methods were used for this comparison. Student ‘t’ test was used for comparison of means. a) Using 90mm plate- Settle plate

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Empty theatre Table 4: Air contamination in empty theatres – settle plate method Theater

Mean No. of CFU/plate

Std. Deviation

Std. Error Mean

CV

2.650

2.7271

0.6818

LAF

0.825

0.5651

0.1413

T=2.621

df =30

p=0.014

Contamination observed by settle plate before surgery in the two theatres appears to be different with mean CFU/plate of 2.650 and 0.825 in the conventional and laminar theatres respectively. This difference is proven to be a true difference by using student ‘t’ test, with a ‘p’ value of 0.014.

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During use Table 5: Air contamination during use – settle plate method Theater

Mean No. of CFU/plate

Std. Deviation

Std. Error Mean

CV

5.088

3.8623

0.9656

LAF

1.425

0.7335

0.1834

T=3.726

df=30

p=0.001

During use also the difference in contamination levels in the two theatres as measured by settle plates is significant with a ‘p’ value of 0.001, by t test. b) Using sampler -

Empty theatre Table 6: Air contamination in empty theatres – Slit-Sampler Theater

Mean No. of CFU/plate

Std. Deviation Std. Error Mean

CV

68.44

25.266

6.317

LAF

2.50

1.549

.387

T=10.41

df =30 p