Which of the following is a reason for a sample to be rejected by QNS?

The reason for a sample to be rejected due to QNS, which stands for "Quantity Not Sufficient," is primarily associated with insufficient volume of blood. When a sample does not meet the required volume or amount needed for accurate testing, it cannot be processed, which is the direct definition of QNS. This is crucial because many laboratory tests require specific volumes to ensure accurate and reliable results; thus, if the sample does not contain enough blood, it is automatically deemed unusable.

In contrast, incorrect labeling could lead to issues with patient identification and result reporting, but it does not affect the physical quantity of the sample. Similarly, using a light blue tube with a 7:1 ratio pertains to specific procedural standards for blood collection but does not inherently relate to QNS. An expired collection tube may also lead to sample rejection, but it is due to concerns about the integrity of the test rather than the volume of the sample itself. Therefore, the core reason related to QNS is the insufficient volume of blood in the sample.