Sample storage, volume and pretreatment

Sample treatment should be considered part of the analysis program. Some instruments require blood samples that are to be aspirated by the sampling probe to be haemolysed, whereas others simply need centrifuged samples. 

The sample volume should be small, although sufficient to perform all the assays required. The maximum amount of sample to be extracted from critical patients and children is 1 mL. This should be divided Into parts sufficiently large to allow the different analyses required (e.g. glucose, CO2) to be carried out. The chief problems resulting from the use of small sample volumes arise from the lack of homogenization when the sample must be reconstituted or thawed after lyophllizatlon and freezing, respectively. 

The instrument should only carry out the requested tests and use the sample volume required. Some simple Instruments allow the Implementation of analytical procedures requiring as little as 20 (iL of sample and featuring excellent precision, reliability and throughput. Improvements In this respect have been aimed at developing automated instruments performing functions similar to those carried out manually. A reasonable goal In this sense Is the possibility of carrying out twenty freely selected assays with 100 (iL of blood sample, which results In a typical sample volume of 5 iL. Such a small volume 

Carry-over arising from the contact between samples, species and reagents should be kept below detectable levels. Alternatively, a computer can be used to compensate for mutual contamination. A logical and dual solution to this problem involves minimizing contact between surfaces by using disposable pipettes, cuvettes, etc., and carefully washing and drying re-usable labware between samples. 

Some measurements such as enzyme activities must, by definition, be made under strictly preset conditions. Those methods involving dichromatic measurements, excessive dilution of the substrate, erratic control of temperature or undefined or non-standardIzed temperatures, or non-preset zero time may yield results consistent with those provided by standard methods, but be considered inadequate because of the lack of suitable material for calibration. Instruments used to implement these methodologies not following the prescribed procedures may be fatal to the patient s treatment. 

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