COMPARISON OF SDR PRESUMED BY AAMI AND THE RADIO RESISTANCE DISTRIBUTION OF THE MICROBIOLOJICAL
POPULATIONS EXIST ON THE TURKISH MADE SINGLE USE SYRINGES
T.KONAC. G.SIYAKUS, M.BASGUL, Z.UNAL Turkish Atomic Energy Authority; ANRCAAS
INTRODUCTION
The concept of radiation sterilization of medical devices was introduced in Turkey 10 years ago through the irradiation facilities in Ankara and Istanbul. It has been preferred by international community due to technological advantages,such as freedom from chemical residues, safer sterility assurance, etc.. This technology is regulated by ISO 11137 (1), EN 552 (2) standards and their references.
These two standards apart from, displaying minor differences, are based on dose setting methodologies ruled by AAMI (Association for Advancement of Medical Instrumentation) (3). Through cooperative studies of Turkish Standards Institute (TSE) and Food Irradiation and Sterilization Department of ANRCAAS (Ankara Nuclear Res. Center in Agric.& Animal Science), a national standard, TS-EN 552 (4) was published in 1996. This standard is an identical version of EN-552, which is the obligatory standard of European Community. All methods are based on a probability model for the inactivation of microbial populations. This model postulated that the bioburden is expressed as a mixture of homogeneous populations, each of which behaves in a ” D10” fashion, given by the following table of SDR (Standard Distribution of Resistance) (5).
Dio
kGy 1.0 1.5 2.0 2.5 2.8 3.1 3.4 3.7 4.0 4.2
Prob. .6 5 4 8 7 .2 2 4 9 3 .0 6 3 0 2 .0 .3 1 7 9 .0 1 2 1 3 .0 0 7 8 6 .0 0 3 5 0 .00111 .0 0 0 7 2 .0 0 0 0 7
Table 1. Standard Distribution of Resistances used in AAMI Dose setting Methods.
The scope of this study was to compare the radio-resistance distribution of the microbial population on the Turkish single use syringes and SDR.
MATERIAL
The disposable syringes from three firms in three geographical regions ( West,Central and East Anatolian Regions ) in Turkey were evaluated. Procurement of product samples was carried out 10 items of each of 3 product lots of each company during 3 months to get a total of 270 samples ((10x3x3)x3 =270).
Measurement method of absorbed dose
Red 4034 dosimeters were used to measure the absorbed dose. Irradiator
The irradiators were carried out in laboratories aimed (Russian made Isslovately type) Co-60 gamma irradiator in ANRCAAS. The dose rate of the irradiator was 4.5 kGy/h in May 1998. The dose rate was measured by using fricke reference standard dosimeters.
Validation of Method
The accuracy of the method of D10 determination was checked using strains of Bacillus pumilus (E601) and Bacillus megaterium (ATCC 8245) spores.
METHODS
Our method had four steps;
l.Determination o f bioburden and Recovery factor,
90 samples were tested every month for a period of three months. Firstly each item and 100 ml of washing solutions were agitated by the use of a shaker for 20 min. After agitation, washing solution was filtrated with membrane filter (pore size; 0.45pm). This procedure was repeated five times and first washing solution was divided into three equal parts for detection of anaerobic, aerobic and fungi counts. All filters were placed on tryptone-glucose-yeast extract agar plates (Plate Count Agar, Oxoid CM325). The plates for anaerobic (within anaerobic jar) and aerobic were incubated for 3 days at 32oC and the plates for fungi were incubated for 7 days at 25oC. The colony forming units were counted. Average microbial contaminants were detected as total microbial numbers divided by total sample numbers. Recovery factor was detected as the total number of first washing divided by the total number of all washing.
2.Simple classification o f microorganisms and than stored o f each culture,
After Gram’s and spore staining, microorganisms were classified with regard to morphology by microscopic observation. All cultures were stored at 4oC for using the following steps.
incubated at 35oC for one day. All plates containing colony between 30 and 300 were counted. And the dilution tubes having a population of 107 viable cells per ml were selected for using in the following step.
4.Determination o f radiosensitivity,
In this step, aluminium foil envelopes containing three strips of drying papers were prepared and then were sterilized for using in irradiation. 0.01mL of suspension (107) was dropped onto strips and packed individually of small envelopes. These envelopes were irradiated with incremental doses (3, 6, 9, 12, 15,18, 21, 24 kGy). After irradiation, each strip was placed in a tube containing 2mL TSB and was incubated at 32 Co for 14 days. The growth (positive) and non-growth (negative) microorganism in tubes were identified and the D10 value was calculated by using the following equation (5).
D10 = Dfp / log (initial no.cells / tube)-(log ( ln (3/no.negative at DFP)))
Where DFP is the highest dose in the incremental series for which the set of three tubes registered 1 and 2 positives.
RESULTS
Table 2.Bioburden and Recovery factor (RF) results of three firms for first month. Washing procedure was validated due to getting the RF values higher than 0.75.
Lots number Recovery factor Average bioburden
A 1 0.86 2.9 2 0.87 31.1 3 0.85 7.4 B 1 0.94 14.0 2 1.0 0.3 3 0.85 3.9 C 1 0.75 1.2 2 1.0 2.7 3 - No growth
Table 3. D10 values of total 50 isolates for three firms(A-27, B-14, C-9)
Firm Isolate DiokGy Firm Isolate DwkGy
A M1 1.63 B Fe1 0.89 M3 0.42 Fe2 2.82 M4 0.42 Fe3 0.46 M5 0.44 Fe5 2.64 M6 0.52 Fe6 0.42 M9 1.44 Fe8 1.99 M11 0.34 Fe10 1.91 M13 0.48 Fe11 0.42 M13 0.75 Fe12 3.60 M19 0.35 Fe13 -M20 0.83 Fe14 1.73 M21 0.20 Fe16 2.02 M22 0.52 Fe17 3.84 M23 1.66 Fe19 0.40 M24 0.86 M26 0.49 M27 1.46 M2 8 0.36 M29 0.41 C T1 1.27 M30 0.21 T2 1.0 M31 1.09 T3 2.44 M32 0.38 T4 0.51 M3 3 1.27 T5 0.41 M34 0.36 T6 0.41 M3 5 0.62 T8 0.44 M3 6 0.49 T9 0.91 M3 7 0.38 T10 0.41
DISCUSSION
SDR presumed by AAMI and the radio-resistance distribution of the microbial population on the Turkish single use syringes was compared with F-statistics test and the differentiation have been importance for the two distributions of resistances (figure 1, table 4). The results showed that, the doses selected by B1 method for the injectors were lower than the doses required by the real population in order to achieve the targeted SAL; hence the B2 method was shown as a preferable method for the routine dose setting studies.
This was the first experimental study in this area, but the aim is to expand the scope of this study to various disposable medicals. In the future, the number of companies will be increased to obtain more realistic data on the distribution of radioresistances of all medicals, which will provide an applicable information related to the subject.
Table-4: F-Test for SDR and Experimental Data SDR Experimental Data Mean 0,1 0,1 Variance 0,042739 0,009353 Number of Survey 10 10 Degree of freedom 9 9 F 4,569515 P(F<=f)one-tail 0,016799 Critical of F Value 3,178897 REFERENCES
1) ISO 11137 Sterilization of health-care products - Requirement for validation and routine control - Radiation sterilization (Reference number: ISO 11137:1995(E).
2) EN 552 Sterilization of medical devices - Validation and routine control of sterilization by irradiation, June 1994, European Standard.
3) Process Control Guidelines for Gamma Radiation Sterilization- AAMI RS-3/84,5 March 1984.
4) TS-EN 552 Tibbi Gereqlerin Sterilizasyonu-Isinlama ile Sterilizasyonda Rutin kontrol ve Geqerliligi, 1996, Turk Standard.
