The McMaster Technique - Practical Task

Arlo Guthrie — Mon, 20 Apr 2009 13:05:48 GMT

Current Revision posted to Revision Guide For Student Nurses - Part 2 by Arlo Guthrie on 4/20/2009 1:05:48 PM

INTRODUCTION
The McMaster Technique is the standard quantitative method used to determine the quantity of round worm eggs per gram of faeces with the use of a special counting chamber known as the McMaster slide.

It must be noted that tapeworm eggs cannot be detected by this method since they do not float in the saturated salt solution used in this test.

Coccidia oocysts may be present, but they float to the surface very slowly. Therefore it is necessary to wait 10 minutes after transferring the solution to the counting chamber.

EQUIPMENT

Protective apron, gloves and mask.
Fresh faecal sample.
Wide mouthed screw-top glass jar.
Saturated salt solution.
Glass beads (for mixing).
Sieve.
Pipette.
McMaster slide.
Microscope.

METHOD

Put on protective apron, gloves and mask; ensure that long hair is tied back and any jewellery removed.
Place 2g of faeces in the screw-top glass jar.
Add 30mls of saturated salt solution.
Add glass beads, replace cap and agitate vigorously in order to break up the sample.
Use a pipette to fill both counting chambers of the McMaster slide (it may be necessary to sieve the suspension if faecal material blocks the pipette).
Check the microscope is safe to use - examine the plug and power cable for damage, then switch on 7) Place the McMaster slide on the stage, and use low power to observe the field for eggs.
If there are more than 1 type of egg, count each separately.
Multiply the number of eggs by 50 to give the estimated quantity of eggs per gram; this is not an accurate test - but does not need to be; an approximate idea of the level of infestation is all that is necessary since all worms are undesirable and need to be eliminated regardless of the amount present.

NB. It is sometimes preferable to centrifuge the faeces suspension prior to transferring it to the McMaster slide. In such cases the supernatant is discarded following centrifugation, the packed sediment is emulsified and the sample topped up with further saturated salt solution. The centrifuge tube is then inverted to even out the suspension and spun down again. The counting chambers are then filled.