Polarisation microscopy increases the sensitivity of hemozoin and Plasmodium detection in the histological assessment of placental malaria.

The histological study of the placenta is useful in the diagnosis of malaria during pregnancy. However, the scarcity of parasites and pigment in many malarial infections renders their identification difficult. We have tested the accuracy of standard and polarisation microscopy in the evaluation of 500 placental specimens from an area of high malarial endemicity in Tanzania. Standard microscopy showed a low sensitivity (50.3% for parasites, 40.5% for pigment), due to poor detection rates in cases with scant parasites (12.7% for <1%; 97.8% for >5% parasitised erythrocytes, P < 0.001 ) or minimal pigment deposition (42.4% versus 84.5% when severe, P < 0.001 ). The use of polarisation microscopy significantly increased the sensitivity of detection of pigment to 100% and parasites to 98.1% because of the marked birefringence of hemozoin present in mature stage parasites which accumulate in the placenta. Formalin pigment shares many properties with hemozoin, but the use of neutral buffered formalin prevented the formation of formalin pigment in placentas even after long periods of fixation. In conclusion, polarisation microscopy is a simple tool that markedly increases the sensitivity of the detection of malaria infection in the placenta and has good specificity when used on tissues fixed in neutral formalin. This method can be useful to investigators working in the malaria field.