Yes, you flush an IV line with saline before giving medication. This is a standard safety step that clears the tubing, confirms the IV is working properly, and prevents drugs from mixing inside the line. Flushing also happens after the medication is given, making it a before-and-after process every time.
Why Flushing Matters
Flushing serves three purposes at once. First, it cleans residual fluid or medication out of the catheter’s internal channel. Second, it confirms the line is open and flowing correctly, which is called checking patency. Third, it prevents leftover traces of one drug from mixing with the next one inside the tubing.
That last point is more dangerous than it sounds. When medications with different pH levels meet inside an IV line, they can form crystals, particles, or cloudy precipitates. These solid fragments can block the catheter or, worse, enter the bloodstream. In one study of drug incompatibilities in a hospital intensive care unit, nearly 69% of incompatibility events involved two medications given by direct injection through the same line. Common culprits included acid-suppressing drugs mixed with anti-nausea medications, which produced visible turbidity and yellow discoloration, and certain antibiotics combined with other drugs that formed white or reddish-brown precipitates. A saline flush between medications eliminates this risk by clearing the line completely.
The SAS Technique
The standard method for flushing around a medication is called SAS: Saline, Administer, Saline. It works like this:
- Saline flush first. A prefilled syringe of normal saline (0.9% sodium chloride) is connected to the IV port. Before pushing saline in, the plunger is pulled back gently to check for blood return, which confirms the catheter is sitting correctly in the vein. If the saline flows smoothly with no swelling, pain, or resistance, the line is good to use.
- Administer the medication. The port is cleaned, the medication syringe is attached, and the drug is pushed in at the recommended rate.
- Saline flush again. A fresh saline syringe flushes the line after the medication. This post-medication flush is given at the same rate as the drug itself, because any medication still sitting in the tubing will enter the bloodstream during the flush. Pushing it too fast could deliver the remaining drug faster than intended.
You may also hear about a longer version called SASH: Saline, Administer, Saline, Heparin. The final heparin step was once common for locking IV lines between uses, but current evidence-based practice no longer recommends heparin for peripheral IVs. Normal saline alone is now the standard for maintaining peripheral line patency.
How Much Saline Is Used
For a standard peripheral IV (the small catheter in your hand or arm), the typical flush volume is 5 mL of normal saline. For central venous catheters, including PICC lines and implanted ports, the standard is 10 mL. These volumes are enough to clear at least twice the internal volume of the catheter and any attached tubing or connectors.
After thicker substances like blood products, IV nutrition, or contrast dye, a larger flush of 20 mL is recommended because these fluids coat the inside of the tubing more heavily and are harder to clear. The Infusion Therapy Standards of Practice, published by the Infusion Nurses Society and most recently updated in 2024, recommend using a 5 to 10 mL syringe for flushing and employing a technique called pulsatile flushing, where you push saline in short bursts rather than one steady push. The start-stop action creates small turbulent swirls inside the catheter that are more effective at dislodging residue clinging to the walls.
Heparin vs. Saline for Locking Lines
Locking is different from flushing. A flush clears the line before and after medication. A lock is the solution left sitting inside the catheter between uses to keep it from clotting shut. For peripheral IVs, saline locks have replaced heparin locks as the standard of care.
For central lines, the picture is slightly more nuanced. A large Cochrane review found that heparin locks resulted in roughly 77 occlusions per 1,000 catheters compared to about 103 per 1,000 with saline alone. But the quality of that evidence was rated very low, and heparin made no meaningful difference in how many days the catheter lasted (about 9 days either way). Because heparin carries its own risks, including bleeding complications and rare allergic reactions, many hospitals now use saline-only protocols for central lines as well, reserving heparin for specific situations.
What Happens Without Proper Flushing
IV catheter failure is surprisingly common. In one clinical trial, 46% of peripheral IVs failed before they were no longer needed, and the two most frequent reasons were occlusion (the line getting blocked) and phlebitis (inflammation of the vein causing pain and swelling), each accounting for 14% of failures. While flushing alone doesn’t eliminate all catheter problems, skipping a flush allows blood to back up into the catheter tip and clot, or lets incompatible drug residues crystallize and plug the line.
Researchers have tested whether flushing more often (every 6 hours versus every 24 hours) or using larger volumes (10 mL versus 3 mL) significantly reduces failure rates. Neither variable on its own showed a statistically significant difference in that trial, which suggests that consistent flushing matters more than the exact volume or schedule. The takeaway: doing it reliably every time, before and after each medication, is what keeps lines functioning.
What the Flush Feels Like
If you’re on the receiving end, a saline flush usually produces a cool sensation traveling up your arm, and some people notice a mild salty taste in their mouth almost immediately. This is normal. What isn’t normal is pain, burning, or visible swelling near the IV site during a flush. These signs suggest the catheter may have shifted out of the vein, and the nurse will stop and assess before continuing. The flush step is actually designed to catch exactly this kind of problem before medication goes in, so it doubles as a safety check every time it’s performed.