Variability of single cholesterol measurement

Normal cholesterol levels are from 3.5 to 6.5 mmol per litre, but ideally levels should be 5.0 mmol per litre or below, since the distribution in the population probably covers an unhealthy population.

Broadly, the ideal levels for the various cholesterol measurements are:

Total cholesterol less than or equal to 5mmol/L

Cholesterol:HDL ratio less than or equal to 4

LDL cholesterol less than or equal to 3mmol/L

HDL cholesterol greater than or equal to 1mmol/L

Note that the cholesterol measurements must be considered in context of primary and secondary prevention of coronary heart disease (CHD). See menu items.

Notes:

• single cholesterol measurement whilst on lipid lowering treatment
  • a single cholesterol reading may under- or over-estimate a person's true average cholesterol level by up to 14%, at least in the first few years of treatment. Therefore, if a clinician has checked that the patient is taking their medication, s/he should be wary of intensifying lipid-lowering treatment in response to a single result
    • data from the LIPID trial were analysed to see how lipid levels vary between patients and over time. The authors found that the 95% confidence intervals on a single cholesterol level measurement were +/-14% of the person’s true average cholesterol
      • in practical terms, this means that for 95 people out of 100 with a single cholesterol measurement of 5.0 mmol/L, their true average cholesterol level will be between 4.3 mmol/L and 5.7 mmol/L
      • measurements were particularly misleading if the initial cholesterol level on treatment was low. In patients with an initial cholesterol of 4.5 mmol/L, even after five years of treatment, a test result greater than 5 mmol/L was just as likely to be incorrect as correct

Reference:

• 1.Glasziou PP et al. Monitoring cholesterol levels: measurement error or true change? Ann Intern Med. 2008 May 6;148(9):656-61
• 2. MeReC Monthly No.7 October 2008.