In the analytical request process, unequivocal identification of the patient, the requested tests, and the requesting physician is necessary. Complementary information such as age, sex, race, dietary habits, toxic habits, physical exercise, etc. are basic data to correctly assign reference values and adequately assess the results. The laboratory must have instructions for the correct completion of analytical requests and disseminate them to the centres making the requests.

If the patient does not perform an adequate preparation, different magnitudes may be altered. For example, a minimum fasting period of 8 hours is recommended before blood sample collection to avoid possible variations in triglycerides, cholesterol, and glucose concentrations. However, fasting for more than 12 hours can be detrimental to the results of other analytes: prolonged fasting can increase serum creatinine and decrease proteins and triglycerides. To reduce errors in the patient's preparation phase, it is advisable to have specific protocols for healthcare personnel and informative documents for the patient, when necessary.

Before sample collection, the correspondence between the request and the patient's identity must be verified. It will be checked that the request contains all identification data, and those not properly completed will be rejected. An unequivocal identification system for samples should be used, ideally with barcodes. The identification data of the person performing the collection, the time and date of the collection, as well as any incidents that have occurred, must be recorded. It is necessary to establish protocols that describe the procedures used for sample collection and train the corresponding personnel.

In the case of venipuncture sample collection, the tourniquet application time should be a maximum of 2 minutes. Excessive prolongation can cause hemoconcentration and alter some analytes concentrations. For patients with therapeutic serums or catheters, it is recommended to collect the sample from the arm opposite to the infusion site. If the sample must be obtained through a catheter, it is recommended to discard the amount of blood equivalent to twice the volume of the catheter beforehand.

The laboratory must ensure that the collection and transport materials have the appropriate characteristics according to the type and volume of the sample. Therefore, all the materials used for each type of sample must be validated.

In the case of blood collection tubes, a specific order of tube filling is recommended to minimize the risk of contamination with the additives they contain. Contamination by anticoagulants should be avoided, knowing the compatibility of each type of sample tube with the analytes to be measured. For analyses on whole blood, a tube with anticoagulant must be used, and the sample must not be centrifuged. If blood is centrifuged in a tube with anticoagulant, plasma is obtained. The possible interferences of the anticoagulant used on the analytes to be measured must be considered. Serum is obtained by centrifuging the blood sample in a tube without anticoagulant.

The transport of samples must be carried out under appropriate conditions. The main factors to consider are transport time, temperature, agitation, exposure to light, placement of samples within the transport container, type of packaging, and identification. It is advisable to have a unified sample transport system between the different collection points and the laboratory. Refrigerators that allow temperature recording throughout the journey are recommended. Care should be taken to avoid excessive agitation that could cause haemolysis of blood samples.

Depending on the type of sample, the analyte to be measured, and when the measurement will be performed, samples must be preserved under specific conditions. In some analyses, it is crucial that the sample is handled under specific temperature conditions from collection to measurement. Each laboratory is recommended to establish the necessary protocols for sample conservation, based on available data on the stability of the measured analytes.

Following specific work protocols established by the laboratory, samples must undergo preparation procedures prior to analysis, when necessary, according to the nature of the sample and the requested analysis. Once prepared, they must be preserved until the moment of analysis under appropriate conditions, avoiding cross-contamination.

The laboratory must review the samples and corresponding requests and record anomalous samples or those that do not meet established criteria. It must alert of incorrect or incomplete sample or request identification. It must also alert of inadequate sample or insufficient volume for the request. Some examples of inadequate samples are:

Coagulated samples

This may be due to slow collection, incorrect mixing of the anticoagulant with the sample, or a defect in the anticoagulant itself. This problem invalidates the results of hemogram, blood gas analysis, and coagulation.

Haemolysed samples

Haemolysis can occur due to excessive tourniquet application time, collecting blood with a syringe and introducing it with too much pressure when filling the tubes, centrifuging blood before complete coagulation, or refrigerating the serum tube before clot retraction. This problem increases the concentrations of several magnitudes, notably LDH, ALT, AST, CK, and potassium.

Lipemic samples

Lipemia is the presence of turbidity in serum or plasma due to increased lipoprotein concentration, either because the patient did not follow the recommended fasting or due to metabolic diseases. Lipemia causes elevated results for analytes whose determinations are based on absorbance at the same wavelengths at which lipid particles also absorb light. For example, it can increase the measured concentration of albumin, calcium, and phosphate.

Icteric samples

Jaundice is an abnormal yellowish coloration due to an excess of bilirubin. Icteric plasma or serum can interfere with analytical results of albumin, cholesterol, triglycerides, glucose, and total proteins.

Samples with drug interferences

Some drugs can produce interferences in the measurement procedure of various analytes. For example, levodopa increases the concentration of creatinine, glucose, and transaminases. Oral contraceptives increase erythrocyte sedimentation rate, glucose, alkaline phosphatase, lipase, and triglycerides concentrations, and decrease cholesterol and protein concentrations. Therefore, it is important for the laboratory to have the patient's clinical information when necessary.

Centrifugation of the blood sample is an important factor to consider for samples arriving from an external collection centre to the laboratory without centrifugation. Ideally, centrifugation should occur within two hours of blood collection. Delay in centrifugation can result in prolonged contact of analytes with cells, which could produce a degradation of the analytes (proteolytic activity), a decrease in their concentration due to cellular metabolism (glycolysis), or an increase due to their diffusion out of cells (potassium).

After processing the samples, they should be stored for a time that ensures that a problem occurring during processing or affecting the interpretation of obtained results can be resolved by recovering the sample for reanalysis, and also to allow a possible request for repetition or extension of analytical tests.

The laboratory must maintain effective communication with requesting centres and collection centres. The laboratory must have documented protocols detailing each phase of the pre-analytical process. All involved personnel must know them and receive appropriate training. The laboratory must keep records of performed activities, including all detected incidents, their analysis, and actions taken for resolution. It is highly advisable for the laboratory to establish performance indicators for the pre-analytical process, analyse their evolution, and implement consequent improvement actions.