Underfilled vacuum blood collection tubes can potentially affect the isoenzyme level of lactate dehydrogenase (LDH) due to several reasons:

Hemolysis: Underfilled tubes may increase the risk of hemolysis during blood collection or transportation. Hemolysis, the rupture of red blood cells, can release cellular contents, including LDH, into the plasma or serum. This release can artificially elevate LDH levels, affecting the accuracy of LDH isoenzyme measurements.

Sample Dilution: Underfilled tubes may contain less anticoagulant or clot activator, leading to insufficient blood volume for proper mixing. Inadequate mixing can result in sample dilution or improper clotting, potentially affecting the accuracy of LDH measurements and isoenzyme analysis.

Incomplete Fill: Underfilled tubes may not provide sufficient blood volume for proper sample analysis, particularly in automated laboratory systems. Incomplete filling can lead to inaccurate measurements or incomplete testing, affecting the reliability of LDH isoenzyme results.

Clot Formation: Inadequate filling of blood collection tubes may result in improper clot formation or partial clotting. Clots can trap cellular components, including LDH, affecting the distribution of LDH isoenzymes in the sample and potentially leading to erroneous results.

The paper you cited concluded that the differences were due to changing rates of the fluid velocity as the tube filled.

Underfilling the vacuum blood collection tube may cause the blood sample to be diluted, which may affect the LDH concentration and possibly distort the relative amounts of the LDH isoenzymes. Moreover, Underfilling of tubes leads to a higher proportion of blood aspirated with high velocity and relevant elevations in LD (Neuwinger et al., 2019). This could result in inaccurate interpretation of test findings, making it challenging to make precise diagnoses or track the progress of diseases

Overall, insufficiently filled vacuum blood collection tubes may result in imprecise assessments of the levels of the enzyme LDH, hence influencing the interpretation of diagnostic tests and the handling of patients. In order to get accurate test results, it's important to make sure the right blood collection procedures are used.

Reference:

Neuwinger, N., Büschenfelde, D. M. Z., Tauber, R., & Kappert, K. (2019). Underfilling of vacuum blood collection tubes leads to increased lactate dehydrogenase activity in serum and heparin plasma samples. Clinical Chemistry and Laboratory Medicine, 58(2), 213–221. https://doi.org/10.1515/cclm-2019-0616