Thick smear

A thick smear concentrates the parasites 10 to 25 times. It is rather more difficult to interpret than a thin smear preparation and often does not permit species identification.

A thick smear contains no intact red blood cells (haemolysis due to the distilled water used in the staining). If a thick smear is positive, a thin smear should be examined. Sometimes parasitaemia is estimated in a thick smear and expressed as +, ++, +++. This form of record is of course quite subjective and confusing and is best avoided. If the thick smear is positive, it is then best to count the percentage of parasited cells in a thin smear preparation. The parasitic density can also be roughly determined in a thick smear, by counting the number of parasites per 200 leukocytes and multiplying this by 30. It is assumed that on average there are 6000 leukocytes per µl blood and that there is one leukocyte per 500 red blood cells. For example: 5 parasites per leukocyte (1000 parasites for every 200 leukocytes) corresponds to a density of 30,000 parasites per µl. Roughly 30,000 parasites per µl corresponds to a parasitaemia of 1% (a moderately anaemic person). If the thick smear is found to be negative in a reliable laboratory, and if there is nevertheless strong suspicion of malaria, the test is repeated every 12 hours for 48 hours. One great disadvantage of the thick smear method is that reliable technical expertise is needed which should be monitored (e.g. quality control). The argument that a lab technician has carried out the test for years and thus has plenty of experience, is absolutely no guarantee of quality or reliability. The test also requires plenty of time if the parasitaemia is low, or before a negative result can be concluded.

Category: Medicine Notes