October 12, 2021: Pharmaceutical Interview Questions and Answers
#FiltersForSterilityTesting #MembraneFiltrationMethodForSterilityTesting #SterilityTestingForAqueousSolutions #SterilityTestingForSolubleSolids #SterilityTestingForOilsAndOilySolutions #SterilityTestingForOintmentsAndCreams
1. Types of filter preferable for sterility testing using filtration method.
Cellulose nitrate filters: For aqueous, oily and weakly alcoholic solutions
Cellulose acetate filters: Strongly alcoholic solutions
2. What is preferred filter diameter for sterility testing using filtration method?
50 mm in diameter
3. Explain Membrane filtration method for sterility testing?
● The filtration apparatus and membrane are sterilized by appropriate means.
● Use membrane filters with nominal pore size not greater than 0.45 μm and diameter 50 mm.
● Solution to be examined shall be filtered under aseptic conditions.
● Aseptically remove the membrane and transfer to the medium.
Method for aqueous solutions:
● Transfer a small quantity of a suitable, sterile diluent such as a 1 g/l neutral solution of meat or casein peptone pH 6.9 to 7.3 onto the membrane in the apparatus and filter.
● Use diluent containing suitable neutralizing substances and/or inactivating substances in the case of antibiotics.
● Transfer the contents of the container or containers to be tested to the membrane or membranes, if necessary after diluting to the volume used in the method suitability test with the chosen sterile diluent but in any case using not less than the quantities of the product to be examined prescribed in Table-1.
● Filter immediately.
● If the product has antimicrobial properties, wash the membrane not less than three times by filtering through it each time the volume of the chosen sterile diluent used in the method suitability test.
● Do not exceed a washing cycle of five times 100 ml per filter, even if during method suitability it has been demonstrated that such a cycle does not fully eliminate the antimicrobial activity.
● Transfer the whole membrane to the culture medium or cut it aseptically into two equal parts and transfer one half to each of two suitable media.
● Use the same volume of each medium as in the method suitability test.
● Alternatively, transfer the medium onto the membrane in the apparatus. Incubate the media for not less than 14 days.
Soluble solids:
Use for each medium not less than the quantity prescribed in Table 3 of the product dissolved in a suitable solvent such as the solvent provided with the preparation, water for injections R, sodium chloride (9 g/l) TS or peptone (1 g/l) TS1 and proceed with the test as described above for aqueous solutions using a membrane appropriate to the chosen solvent.
Oils and oily solutions:
● Use for each medium not less than the quantity of the product prescribed in Table 2.
● Oils and oily solutions of sufficiently low viscosity may be filtered without dilution through a dry membrane.
● Viscous oils may be diluted as necessary with a suitable sterile diluent such as isopropyl myristate R shown not to have antimicrobial activity in the conditions of the test.
● Allow the oil to penetrate the membrane by its own weight then filter, applying the pressure or suction gradually.
● Wash the membrane at least three times by filtering through it each time about 100 ml of a suitable sterile solution such as peptone (1 g/l) TS1 containing a suitable emulsifying agent at a concentration shown to be appropriate in the method suitability test, for example polysorbate 80 at a concentration of 10 g/l.
● Transfer the membrane or membranes to the culture medium or media or vice versa as described above for aqueous solutions, and incubate at the same temperatures and for the same times.
Ointments and creams:
● Use for each medium not less than the quantities of the product prescribed in Table 2.
● Ointments in a fatty base and emulsions of the water-in-oil type may be diluted to 1 per cent in isopropyl myristate R as described above, by heating, if necessary, to not more than 40 °C.
● In exceptional cases it may be necessary to heat to not more than 44 °C.
● Filter as rapidly as possible and proceed as described above for oils and oily solutions.
Table: 1 – Liquids sample
| Minimum quantity to be used for each medium Quantity per container | Minimum quantity to be used for each medium unless otherwise justified and authorized |
| • less than 1 ml | The whole contents of each container |
| • 1-40 ml | Half the contents of each container but not less than 1 ml |
| • greater than 40 ml and not greater than 100 ml | 20 ml |
| • greater than 100 ml | 10 per cent of the contents of the container but not less than 20 ml |
| Antibiotic liquids | 1 ml |
Table: 2 – Insoluble preparations, creams and ointments sample
| Minimum quantity to be used for each medium Quantity per container | Minimum quantity to be used for each medium unless otherwise justified and authorized |
| Insoluble preparations, creams and ointments to be suspended or emulsified | Use the contents of each container to provide not less than 200 mg |
Table: 3 – Solids sample
| Minimum quantity to be used for each medium Quantity per container | Minimum quantity to be used for each medium unless otherwise justified and authorized |
| • less than 50 mg | The whole contents of each container |
| • 50 mg or more but less than 300 mg | Half the contents of each container but not less than 50 mg |
| • 300 mg – 5 g | 150 mg |
| • greater than 5 g | 500 mg |
Reference: WHO, Document QAS/11.413 FINAL, March 2012
