Q and A

Units of measure – mm3, mL, μL etc

CD4 and viral load are measured in different volumes.

CD4 counts are measured in cells per cubic millimetre = cells/mm3

A cubic millimetre (mm3) = mm3  = mm= 1 mm x 1 mm x 1 mm.

= microlitre (µL).

= a metric measure of volume or capacity equal to a cube that is 1 millimeter on each edge.

Sometimes CD4 is written as cells/µL = cells/microlitre = millionth of a litre.

Sometimes CD4 results are written as cells / log x 106 (ie x 10 x 10 x 10 x 10 x 10 x 10).

If your CD4 count is 200, this can be written:

  • 200 cells/mm3
  • 200 cells/µL
  • 200 x log10 6 cells/L

Viral load is measured in copies per millilitre = copies/mL

A cubic centimetre (cm3) = millilitre (mL)

= one cubic centimetre = 1cm x 1cm x 1 cm = cm3 = cm3.

= one mL  = 1000 µL.

= a metric unit of volume equal to one thousandth of a litre.

Viral load of 20,000 is written:

  • 20,000 copies/mL
  • 20,000 c/mL HIV RNA.
  • This is sometimes written 2.0 x 10 (x). The letter “x” can be any number from 1 to 8 to show whether you are multiplying by 10, 100, 1000, 10 000, 100 000 etc. In this case it would be 2.0 x 104 – ie 2.0 x 10,000.
  • Using a log scale this would be 4.3 log10 copies/mL.

Conversion table between viral load as numbers and logs.

1,000 microlitres = 1 millilitre

1,000 millilitres = 1 litre

1,000,000 microlitres = 1 litre

[1 US pint = 0.47 litres]

If there are 5.5 litres of blood in your body:

Q1.       How many millilitres (mL) of blood are in your body?               A.       _______

Q2.       How many microlitres (µL) of blood are in your body?        A.       _______

[Answers: Q1: 1000 x 5.5 = 5,500; Q2: 1000 x 1000 x 5.5 = 5,500,000]

Last updated: 16 September 2016.