The experience of having blood flow suddenly stop during a draw can be frustrating for both the patient and the healthcare professional. This common occurrence rarely indicates a serious issue for the patient, but rather points to a temporary mechanical, physiological, or technical interruption in the process. Interruptions are primarily due to the physical interaction between the needle and the vein, the pressure dynamics of the collection system, or, less frequently, issues with coagulation or equipment.
Needle Position and Physical Obstruction
The most frequent reason for blood flow stopping is a physical obstruction at the needle tip within the vein. The needle’s opening, known as the bevel, must be completely and cleanly positioned within the vein’s lumen for continuous flow. If the bevel rotates or shifts, it can press against the inner wall of the vein, which acts like a seal and blocks the entry of blood into the needle.
This issue often occurs because veins have relatively thin walls, making them flexible and susceptible to movement. Even a slight movement by the patient, or the phlebotomist changing the collection tube, can cause a minor shift in the needle’s angle. The needle may also encounter a venous valve, which can obstruct the bevel and slow or stop the flow. A trained professional can often resolve this by making a subtle, controlled adjustment, such as slightly withdrawing or rotating the needle to move the bevel off the vessel wall or past a valve.
Sometimes, the needle is inserted too far, causing it to pass through the back wall of the vein entirely. When the needle is only partially within the vein, or if it punctures both walls, blood can leak into the surrounding tissue, forming a hematoma and reducing the pressure needed for successful collection. If the needle is not deep enough, it may only partially enter the vein, resulting in a slow, intermittent flow that quickly ceases.
Vein Collapse Due to Vacuum Pressure
Another cause of abrupt flow cessation involves the vein’s physiological response to the vacuum used in modern collection systems, like Vacutainer tubes. These tubes are pre-filled with negative pressure designed to draw a precise volume of blood. This vacuum creates a pressure gradient that pulls blood through the needle. If the pressure exerted by the vacuum is greater than the internal pressure within the vein, the flexible vein walls can flatten around the needle.
This is known as vein collapse, and it effectively seals the needle opening, immediately stopping the blood flow. Patients who are dehydrated, elderly, or have naturally small or fragile veins are more susceptible because their veins may lack the structural integrity to withstand the suction. If a vein collapses, the phlebotomist may attempt to alleviate the pressure by switching to a smaller collection tube, which contains less vacuum, or by temporarily releasing the tourniquet. The vein’s thin structure, which lacks the muscular support of an artery, makes it vulnerable to this pressure imbalance.
Rapid Clotting and Equipment Failure
Less common, but still possible, are issues related to the blood’s immediate coagulation or a fault in the collection equipment itself. In rare instances, a patient’s blood may begin to coagulate very rapidly within the small diameter of the needle or the tubing of a butterfly set. This can cause the blood flow to slow down and then completely stop before the required volume is collected, requiring immediate intervention to prevent a blocked needle.
Equipment problems can also mimic a physiological or positional failure, most notably when the collection tube is defective. The vacuum within an evacuated tube can be compromised if the tube is expired, dropped, or has an imperceptible manufacturing defect, resulting in insufficient suction to draw the blood. A loose connection in the holder or a tube that was not fully engaged onto the inner needle can also prevent the vacuum from being properly applied to the system. If the blood flow stops and the needle position appears correct, replacing the collection tube is often the quickest way to rule out a faulty vacuum as the cause.