Why are these values out of range?
Q: When my child's blood was tested, we got the following results: Lymphocytes 51.0 (range 25 - 50), Monocytes 7.6 (range 1-6) NORMAL: WBC 7.7 (range 4.5-13.5) Neutrophils 38 (range 28 -66) Eosinophils 2.7 Basophils 0.7. What is the implication of these out of range values?
A:Your child’s values are ‘normal’ for age. Any biological parameter that is measured in a population needs a defined physiological or ‘normal’ range. But every haematological test is affected by pre-analytical variables like technique and timing of blood collection, posture of patient and the transport and storage of specimens. Then there are variations in the analytic technique which impact on the test result. But most of these can be standardised. Problem is caused by inherent factors of age, sex, body build, occupation, genetic background, diet, altitude of residence etc., which affect haematological values. All these variables have to be taken into account while defining ‘normal’ values. A reference interval is typically established by assaying specimens that are obtained from individuals that meet carefully defined criteria (reference sample group). It is difficult to be certain in any survey of a population for the purpose of obtaining data from which ‘normal’ ranges are established that ‘normal’ subjects are completely healthy. Haematological values for the normal and abnormal will overlap, and a value within the recognised normal range may be definitely pathological in a particular subject. For this reason the concept of ‘normal ranges’ or ‘normal values’ has been replaced by ‘reference limits’ and ‘reference values’ in which the variables are defined when establishing the values for a reference population. For any test, the range between the reference limits is called the reference interval and ideally each laboratory should establish its own. This is typically established by analysing a minimum of 120 reference samples. If a range of observations is used and sampling is carried on, eventually some observations will be outside it, and the range will get bigger and bigger. To avoid this, a range between two quantiles is used, usually the 2.5 centile and the 97.5 centile, which is called the normal range, 95% reference range, or 95% reference interval. This leaves 5% of normals outside the `normal range', which is the set of values within which 95% of measurements from apparently healthy individuals will lie. In other words, in a statistically normal distribution of data (and most biological analytes have normal distribution), most observations will fall within two standard deviations of the mean with 2.5% below and 2.5% above it. A normal haematological value will be the Mean + 2 S.D. (as defined by the lab).