Background:
(1) In leprosy most of the bacteria are in dermal granulomas.
(2) Bacteria outside of granulomas are rare.
(3) The bacterial load depends on the volume of the granulomas times the density of the bacteria within the granulomas.
Specimen: skin biopsy sectioned for microscopic examination
Parameters:
(1) bacterial index (BI), as measured in the previous section
(2) decimal fraction of dermis in a microscopic field composed of granuloma
decimal fraction of dermis =
= (area composed of granuloma) / (area of dermis)
logarithm of biopsy =
= (BI for biopsy) + LOG10(decimal fraction of dermis)
where:
• The BI is viewed as a logarithmic scale for bacterial density.
• Since the decimal fraction is less than 1 the LOG10 is a negative number. For example, a decimal fraction of 0.1 (10%) has a logarithm of -1.
Interpretation:
• The maximum LIB is 6 (assuming BI of +6 and decimal fraction of 1.0.)
• The minimum LIB is possibly -3.
• An LIB of 1 indicates 100 bacilli per cubic millimeter.
• An LIB of 6 indicates about 10 million bacilli per cubic millimeter.
bacterial load per cubic millimeter =
= 10^(LBI + 1)
