Manual of basic techniques for a health laboratory - libdoc.who.int

154 Manual of basic techniques for a health laboratory●●Formalin, 10% solution (100ml of formaldehyde, 37% solution in 900ml ofdistilled water)Ether (or ethyl acetate).Fig. 4.105 After centrifuging thesuspension, loosen thefatty plug and discardthe supernatantMethod1. Using a wooden applicator, remove a small amount (approximately 0.5g) offaeces from both the surface and the inside of the stool specimen.2. Place the sample in a centrifuge tube containing 7ml of 10% formalin.3. Emulsify the faeces in the formalin and filter into the dish.4. Wash the filter (with soapy water) and discard the lumpy residue.5. Transfer the filtrate to a large test-tube. Add 3ml of ether (or ethyl acetate).6. Stopper the tube and mix well.7. Transfer the resulting suspension back to the centrifuge tube and centrifugeat 2000g for 1 minute.8. Loosen the fatty plug with an applicator and pour the supernatant awayby quickly inverting the tube (Fig. 4.105).9. Allow the fluid remaining on the sides of the tube to drain on to thedeposit and then mix well. Using the pipette, transfer a drop on to theslide and cover with a coverslip.10. Use the ¥ 10 and ¥ 40 objectives to examine the whole of the coverslipfor ova and cysts.It is now common practice to perform all the above steps in a biologicalsafety cabinet. If the extraction system of the cabinet is not fireproof, thesteps involving ether should be done outside the cabinet. Ethyl acetate providesa less flammable alternative to ether.4.5.3 Formaldehyde–detergent sedimentation techniquePrincipleThe formaldehyde–detergent sedimentation technique is an inexpensive, safe andsimple quantitative sedimentation method in which a measured amount of faecesis mixed in formaldehyde–detergent solution of low specific gravity. The suspensionis sieved and is then left undisturbed to allow the ova to sediment under theirown weight. The detergent “clears” the faecal debris in a short time. Followingsedimentation and clearing, the small amount of fine sediment which forms isexamined under the microscope for ova and the eggs are counted to give aquantitative result.Materials and reagents● Microscope● Microscope slides● Commercial test kit, consisting of a conical-based container, a plastic strainer, aPasteur pipette, a beaker and a commercial detergent, diluted 1:50 with distilled water● Formalin, 2% solution (prepared by diluting formaldehyde, 37% solution 1:50with distilled water).MethodDetails of the method as supplied with the kit are as follows:1. Fill the conical-based container to the 10-ml mark with 2% detergent in 2%formalin.