What is a 'normal' blood test, asked a reader last week. Several readers point out that there are so many different types of test, and the laboratory will look only for those specific markers or components in the blood that the doctor has requested. You can check the blood for the presence of parasites or unwanted microbes, and you can check normal blood components for general health, such as homocysteine, cholesterol, or blood sugar levels. One reader has given a very full explanation of the tests and so it is, with thanks, that we pass the question to him. Within the components usually tested for nowadays, orthodox figures are based on averages. So these are at best a guide rather than a diagnosis. Some people can function with lower than average iron levels, which anyway respond poorly to supplementation. However, there are often indications that increasing or decreasing readings are beneficial or harmful - so it is often best to have two or more tests, spaced out over months or years, and see whether the figures are improving - including ratios (for cholesterol components, for example). Because the USA is so dominant, but uses different measurement units, this causes an additional difficulty in understanding the literature. The USA uses milligrams per litre or sub-units thereof; the EU has standardised on moles per litre, usually a subdivision into millimoles and micromoles per litre. There is not one single factor that converts between the two systems. As much of the scientific and popular press reports from both systems, this makes it harder to know whether your personal values are OK or not: for example, fasting blood glucose levels point to a need for action if they exceed 120 in the USA, but 7 in the UK - the former in mg/100ml, the latter in mmol/l. It is thought that the total cholesterol level is of secondary importance to that of the ratio of triglycerides (bad cholesterol) to high-density cholesterol (good cholesterol), underlined by the bad side effects, and low efficacy, of cholesterol-lowering drugs. Ratios of about .5 are considered good; above 2 poor; above 3 dangerous, using mmol/l. Homocysteine is now thought to be a better predictor of cardiovascular health, with lower values within the range of 5 to 15 micromoles/l acceptable.