Jackie,
Are you taling about 24-48 hours to get the test results back, or 24-48 hours before the test is done?
I googled the following phrase ‘inr changes in blood tests with delay after drawing' and there were several interesting hits on the topic, two of which are copied below. The second is the more valuable and scientifcally proven paper.
The American Society for Clinical Laboratory Science
Table 4: Causes of Protime (INR) Variability
• Missed dosage
• Dietary changes: foods high in vitamin K
• Blood collection variations: short draw, clots, high temperature, delay cause high INRs
• Blood collection variations: long draw, prolonged chilling, shaking cause low INRs
Blood collection errors also cause protime changes. The laboratory scientist must ensure that the blood reaches the collection tube "fill line" and must gently mix it within seconds of the time it is collected to make sure it does not clot. "Short draws" or clotted blood both give erroneously high INRs. Blood that is transported at temperatures above 80° Fahrenheit or stored at room temperature for more than 24 hours also causes high INRs. On the other hand, if the scientist overfills the blood collection tube, or the blood is stored in the refrigerator for more than 24 hours, the INR will be falsely low. And if the blood is shaken too vigorously in mixing, its cells may rupture, also causing false low values. If there is any suspicion that a collection error has occurred, the test should be repeated before the dosage is adjusted.
Clinical Chemistry. 2005;51:561-568.)
Preanalytical Variables and Off-Site Blood Collection: Influences on the Results of the Prothrombin Time/International Normalized Ratio Test and Implications for Monitoring of Oral Anticoagulant Therapy
Abstract
Background: The quality of oral anticoagulant therapy management with coumarin derivatives requires reliable results for the prothrombin time/International Normalized Ratio (PT/INR). We assessed the effect on PT/INR of preanalytical variables, including ones related to off-site blood collection and transportation to a laboratory.
Methods: Four laboratories with different combinations of blood collection systems, thromboplastin reagents, and coagulation meters participated. The simulated preanalytical variables included time between blood collection and PT/INR determinations on samples stored at room temperature, at 4–6 °C, and at 37 °C; mechanical agitation at room temperature, at 4–6 °C, and at 37 °C; time between centrifugation and PT/INR determination; and times and temperatures of centrifugation. For variables that affected results, the effect of the variable was classified as moderate when <25% of samples showed a change >10% or as large if >25% of samples showed such a change.
Results: During the first 6 h after blood collection, INR changed by >10% in <25% of samples (moderate effect) when blood samples were stored at room temperature, 4–6 °C, or 37 °C with or without mechanical agitation and independent of the time of centrifugation after blood collection. With one combination of materials and preanalytical conditions, a 24-h delay at room temperature or 4–6 °C had a large effect, i.e., changes >10% in >25% of samples. In all laboratories, a 24-h delay at 37 °C or with mechanical agitation had a large effect. We observed no clinically or statistically relevant INR differences among studied centrifugation conditions (centrifugation temperature, 20 °C or no temperature control; centrifugation time, 5 or 10 min).
Conclusions: We recommend a maximum of 6 h between blood collection and PT/INR determination. The impact of a 24-h delay should be investigated for each combination of materials and conditions.
