A saline flush is a small injection of sterile salt water pushed through an intravenous (IV) catheter to keep it clean and functioning. The solution is 0.9% sodium chloride, meaning 900 milligrams of salt dissolved in every 100 milliliters of water. This concentration matches your blood’s natural salt level, so it won’t irritate your veins or disrupt your body’s fluid balance.
If you have an IV line for medications, fluids, or blood draws, you’ll see nurses use saline flushes regularly. They’re one of the most routine parts of IV care, and if you’re going home with a catheter, you may need to do them yourself.
Why Saline Flushes Are Necessary
Any time blood, medication, or IV fluid sits inside a catheter, residue can build up along the inner walls. Blood proteins and platelets can collect, bacteria can cling to the surface, and leftover medication can crystallize. Over time, these deposits narrow the catheter’s opening and can block it entirely. A blocked catheter means it has to be replaced, which means another needle stick.
Saline flushes solve this by physically washing the inside of the tubing clean. Flushing before and after each medication also prevents two incompatible drugs from mixing inside the line, which could cause a reaction or form particles that enter your bloodstream. Beyond clearing debris, regular flushing with normal saline reduces the buildup of bacteria, proteins, and platelets, which lowers the risk of vein inflammation (phlebitis) at the catheter site.
When Flushes Happen During IV Care
Healthcare providers follow a specific sequence every time they use your IV line. The standard pattern is called SAS: saline flush, then administration of medication or fluid, then another saline flush. The first flush confirms the line is open and working. The second flush clears any remaining medication from the tubing so it all reaches your bloodstream rather than sitting in the catheter.
When blood is drawn through the line, the same logic applies: a saline flush before the draw, the blood collection, then a saline flush after. Blood is thicker than medication, so flushing afterward is especially important to prevent clotting inside the catheter. In some cases, a heparin lock (a small dose of blood-thinning solution) is added at the very end to keep the line from clotting between uses. This extends the sequence to SASH: saline, administration, saline, heparin.
If your IV isn’t actively being used but needs to stay in place (sometimes called a saline lock), it’s typically flushed at least once per nursing shift to keep it open.
How the Flush Actually Works
Nurses don’t simply push saline through in one steady stream. The recommended technique is called pulsatile flushing: a series of short, quick pushes separated by brief pauses. A typical approach is ten small 1-milliliter pushes from a 10-milliliter syringe, each followed by a fraction-of-a-second pause.
This stop-and-start motion creates tiny bursts of turbulence inside the catheter. The turbulent flow is far more effective at scrubbing residue off the inner walls than a smooth, continuous push would be. The swirling action dislodges medication buildup and fibrin (a protein involved in blood clotting) that clings to the catheter surface. When the push stops, the turbulence fades almost instantly, within about a tenth of a second, so the cycle of push and pause keeps generating fresh bursts of cleaning action.
How Much Saline Is Used
The volume depends on your body size and the type of catheter. For adults weighing over 50 kilograms (about 110 pounds), a standard flush is 5 to 10 milliliters. For children between 5 and 50 kilograms, the volume drops to 2 to 5 milliliters. Very small infants under 5 kilograms receive just 1 to 2 milliliters.
These volumes are small enough that they don’t meaningfully add to your fluid intake, but they’re large enough to clear the full length of the catheter. Prefilled syringes come ready to use in standard sizes, so there’s no need to measure or draw up the solution from a vial in most clinical settings.
Saline vs. Heparin for Keeping Lines Open
For years, heparin (a blood thinner) was the go-to solution for locking IV lines between uses. The idea was that a small amount of heparin sitting inside the catheter would prevent blood from clotting and blocking the line. Normal saline flushes were considered a simpler alternative, and clinical research has compared the two extensively.
For standard peripheral IVs, saline alone is now widely preferred. It maintains catheter function without the risks that come with heparin, including a rare but serious allergic reaction called heparin-induced thrombocytopenia. For central venous catheters (longer lines that reach larger veins near the heart), some protocols still use heparin locks, though evidence supports normal saline as equally effective in many situations. The trend in clinical practice has moved steadily toward using saline whenever possible.
What It Feels Like
Most people feel nothing during a saline flush, or at most a brief cool sensation traveling up the arm as the room-temperature solution enters the vein. Some people notice a slight salty taste in their mouth almost immediately, which is harmless. If you feel pain, burning, or see swelling near the IV site during a flush, that’s a sign the catheter may have shifted out of the vein. The flush should be stopped right away so the site can be checked.
Flushing an IV Line at Home
If you’re discharged with a PICC line (a catheter that runs from your arm to a large vein near your heart) or another long-term IV, you’ll likely be taught to flush it yourself. The basic steps are straightforward, but technique matters.
Before touching the catheter’s connection port, clean it by scrubbing firmly with an alcohol or chlorhexidine wipe for at least five seconds, then let it dry completely. Don’t blow on it or wave your hand over it. Attach a prefilled 10-milliliter syringe of preservative-free normal saline to the port. Before pushing any saline in, gently pull back on the plunger to check for blood return. Seeing a small flash of blood confirms the catheter is sitting correctly inside the vein.
If no blood appears, you can still try flushing a small amount of saline slowly while watching closely for resistance, swelling, leaking, or pain. If any of those occur, stop. Never force saline through a catheter that feels blocked. Forcing fluid through a clogged line can push a clot into your bloodstream or rupture the catheter itself.
Once you’ve confirmed the line is clear, use the pulsatile technique: short pushes with brief pauses rather than one continuous push. After flushing, clamp the line while you’re still pressing the plunger forward slightly. This “positive pressure” technique prevents blood from being sucked back into the catheter tip as you disconnect the syringe, which would defeat the purpose of flushing. Clean the port again before capping it.