Pre-Analytical Variables( PDF) D-dimer: preanalytical, analytical, postanalytical variables, and...

A large number of variables can potentially affect the results of a laboratory test and these can be divided into:

A ratio of 1 part Anticoagulant:9 parts blood is used for haemostasis tests.Under-filling of the tube results in an increased ratio of citrate and when the sample is re-calcified, only part of it takes place.If the patient has an elevated Hct, similar results may be seen.

The ratio of 9:1 is maintained if the haematocrit exceeds 0.55.The formula is shown.

The Hct can be used to calculate the volume of 3.2% Trisodium citrate required to maintain the correct ratio of anticoagulant to whole.A sample tube of 5ml and a patient with a haematocrit of 0.65 is the correct ratio.An alternative method for calculating the concentration of 3.2% trisodium citrate to be used as an anticoagulant is shown in the graph below.

The volume of the sample tube should be based on the amount of blood taken in the neonate and not the total amount.Most laboratories and neonatal units accept a degree of prolongation of the coagulation times and most reference ranges are based on this.

The results of specific tests can be influenced by the transport and storage of samples.Delays of more than 4 hours between the collection and processing of samples can affect the results.The shelf-life of samples is affected by the temperature at which they are stored.The release of Phospholipids can lead to the lysis of any remaining platelets.

Alterations to the effect of certain patient factors can be made with the knowledge that they are largely outside the control of the laboratory.

Ensuring that all staff are aware of the pre-analytical variables that may affect the results of any test is a must.References can be found for further information.