Thin blood smear

on 6.10.08 with 0 comments

This shows the presence of undistorted parasites. The thin blood smear permits identification and also calculation of the parasitaemia (% of parasitised red blood cells). This is necessary to start appropriate therapy (P. vivax is treated differently from P. falciparum). If the parasite cannot be identified it is regarded as a P. falciparum as a safety precaution. Mixed infections do occur. For staining, Giemsa is used with a slightly alkaline pH. It is a good habit to prepare the buffer solutions each day in the morning (interaction with CO2 from the atmosphere changes the pH of older solutions). Phosphate buffers are mostly used.

          1. First a stock solution is made of KH2PO4. This is made by placing 9.078 g KH2PO4 in one litre of purified water. This stock solution can be used for weeks if correctly stored (in a closed bottle).

          2. A stock solution of Na2HPO4.2H20 is also made by mixing 11.877 g with one litre of purified water. If one uses the anhydrate (Na2HPO4) in place of Na2HPO4.2H20, only 9.474 g per litre is used. This stock solution can also be used for weeks if correctly stored (in a closed bottle).

          3. To obtain a buffer with a pH of 8, add 5.5 ml of KH2PO4 solution to 94.5 ml of Na2HPO4.2H20 solution and dilute with 900 ml of distilled water. One then has 1 litre of buffered water with a pH of 8. This can then be used for the malaria blood smears for the rest of the day.

          4. For the leukocytic formula it is best to stain with a slightly acid pH of 6.4. For this a different phosphate buffer is used. This requires different ratios. Now 26 ml of the Na2HPO4.2H20 solution is mixed with 74 ml of the KH2PO4 solution and then 900 ml water is added.

An alternative to Giemsa is acridine orange, but the day-to-day use of this technique is quite unpleasant and it is necessary to have a special microscope (ocular filter, halogen light, interference filter above the condenser). [This latter filter restricts transmission to some narrow spectral bands].

Category: Medicine Notes



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