Ridley developed the parasite index (PI) for quantitating amastigotes in skin biopsies from patients with cutaneous leishmaniasis. The author is from the Hospital for Tropical Diseases in London (when it was still on St Pancras Way).
Criteria for a standard section (counting area):
(1) completely occupied by a lesion of cutaneous leishmaniasis
(2) occupies 3 microscopic fields under a 10x objective
(3) cut at 4 microns thin
Number of Amastigotes Per Standard Section
Index
0
0
1 - 9
1+
10 - 99
2+
100 - 999
3+
1,000 - 9,999
4+
10,000 - 99,999
5+
>= 100,000
6+
where:
• This is roughly a logarithmic system (LOG10(number) + 1).
• The peripheral part of a lesion without parasites is ignored.
If the lesion is smaller or larger, the count is adjusted to 3 10x microscopic fields.
A heavily parasitised macrophage in a section may contain 10-20 amastigotes (on average). A 40x objective (high dry) consisting only of such cells has about 5,000 organisms per field. If the lesion is composed entirely of such fields, then it is classified as 6+. A lesion with fewer organisms or less extensive would be 5+.
To read more or access our algorithms and calculators, please log in or register.
