Guideline: the laboratory diagnosis of malaria. General Haematology Task Force of the British Committee for Standards in Haematology

UK National External Quality Assessment Service surveys indicate continuing problems in malaria diagnosis: inaccurate calculation of parasitaemia or failure to estimate it altogether, difficulty distinguishing Plasmodium vivax from P. ovale, reporting malaria parasites when none were present and misidentification of P. falciparum as another species still occur. Therefore, the British Committee for Standards in Haematology Guidelines for the Laboratory Diagnosis of Malaria have been revised. They are intended for use in the UK but may also prove useful in other non-endemic areas. Routine use of thick and thin films is advised for malaria diagnosis. Thick films should be stained using Giemsa or Field stain. Thin films should be stained with Giemsa stain or Leishman stain. Thick films should be examined by two observers, each viewing a minimum of 200 high power fields. If thick films are positive, the species should be determined by examination of a thin film. In the case of P. falciparum or P. knowlesi infection, the percentage of parasitized cells or the number of parasites per microlitre (/μl) should be estimated and reported. Rapid diagnostic tests (RDTs) for malarial antigen cannot replace microscopy but are indicated as a supplementary test when malaria diagnosis is performed by relatively inexperienced staff. Malaria RDTs are negative in babesiosis.

Morphology in the diagnosis of red cell disorders

Despite the advances in automated blood cell counting, the blood film retains a crucial role in the diagnosis of red cell disorders. It is particularly important in haemolytic anaemias and in the differential diagnosis of macrocytic anaemia. However, all cases of anaemia in which the diagnosis is not immediately obvious require a blood film. Blood film examination sometimes provides a definitive diagnosis but more often suggests a differential diagnosis that indicates which further tests are most appropriate. The blood film has the advantage of speed; this is clinically important in any severe anaemia but particularly in acute haemolytic anaemia, thrombotic thrombocytopenic purpura and megaloblastic anaemia. Polycythaemic as well as anaemic patients require blood film examination.