# Units of measurement

This page covers two basic units used to count CD4 cells and viral load:

- cubic millimetre – mm
^{3} - millilitre – mL

And two advanced units often used in studies:

- log value – log
_{10} - percentage – %

## Cubic millimetre – mm^{3}

**A cubic millimetre measures a very small volume. The length, depth and height of the volume are all 1 millimetre (mm) long.**

In scientific papers cubic millimetre are sometimes shown as microlitres (µL) or million parts per litre (10^{6}/L).

CD4 count is measured as number of cells in a cubic millimetre (mm^{3)}. One cubic millimetre is about a drop of blood.

## Millilitre – mL

**A millilitre measures a volume of liquid.**

Viral load is measured as number of copies in a mL.

1,000 mm^{3} = 1 mL. 1,000 mL = 1 litre.

## Log value – log_{10}

**A log value shows a number as a factor of 10.**

Log values make it easier to deal with big numbers.

#### Measuring viral load using logs

Viral load is often measured using a log value.

- A 1 log drop in viral load means the amount of HIV has gone down by 90%
- A 2 log drop means it has gone down by 99%

If your viral load was 20,000 then a 1 log drop equals a 10-fold (10 times) decrease to 2,000. A 2 log drop equals a 100-fold decrease (10 x 10 times) to 200.

- A 1 log drop in viral load can mean different things.

A drop from 20,000 to 2,000 and from 2,000 to 200 are both drops of 1 log.

#### Table: log = number

1 log_{10} = 10 |
1.5 log = 30 | 1.7 log = 50 |

2 log_{10} = 100 |
2.5 log = 300 | 2.7 log = 500 |

3 log_{10} = 1,000 |
3.5 log = 3,000 | 3.7 log = 5,000 |

4 log_{10} = 10,000 |
4.5 log = 30,000 | 4.7 log = 50,000 |

5 log_{10} = 100,000 |
5.5 log = 300,000 | 5.7 log = 500,000 |

## Percentage – %

**A percentage shows a number as a proportion of 100 – the number of parts in 100 parts.**

- 1% = 1 in 100

Percentages are often used to compare proportions from different samples.

CD4 cells are sometimes counted as a percentage – the proportion of lymphocytes (white blood cells) that are CD4 cells. A CD4% of 12.5 usually equals a CD4 count of 200 in adults.

Percentages can be made easier to understand by turning them into fractions:

- 20% = 20 in 100 =
^{20}/_{100}

Sometimes percentages are shown as a decimal fraction of 1 (100% = everything you count = 1). To make a percentage into a decimal fraction, divide it by 100.

- 20% =
^{20}/_{100}= 0.20

#### Table: common percentages and what they mean

**5%**= 0.05 =**1 in 20****10%**= 0.10 =**1 in 10****12.5%**= 0.125 =**1 in 8****16.5%**= 0.165 =**1 in 6****20%**= 0.20 =**1 in 5****25%**= 0.25 =**1 in 4**= a quarter**33%**= 0.33 =**1 in 3**= a third**50%**= 0.50 =**1 in 2**= half**66%**= 0.66 =**2 in 3**= two thirds**75%**= 0.75 =**3 in 4**= three quarters**80%**= 0.80 =**4 in 5****90%**= 0.90 =**9 in 10****95%**= 0.95 =**19 in 20**

#### Percentages can deceive!

Percentages show proportions, not actual numbers. It is sometimes difficult to see the whole picture:

**A study reports:**80% of people using our new drug had an undetectable viral load at the end of the study.**Questions:**how many people completed the study? Was it many people, or only a few? Maybe lots of people dropped out because of side effects!?

The report suggests the new medicine worked for 8 in 10 people who took the drug – when it really worked for 8 in 10 people who completed the study.

**When looking at percentages, remember to ask ‘percentage of what?’**

Last updated: 22 July 2009.