A subcutaneous (SC) injection delivers medication into the fatty layer of tissue (the hypodermis or subcutis) just beneath the skin. This route is chosen for drugs, such as insulin or certain hormones, that require slower, more sustained absorption into the bloodstream compared to injection into a muscle or vein. The angle of needle insertion determines whether the dose reaches the intended fatty layer without penetrating too deeply, ensuring efficacy.
Understanding the Injection Angle
The depth of the subcutaneous tissue varies among individuals and across different body areas, leading to the use of two primary angles for insertion: 45 degrees and 90 degrees. The goal is to deposit the medication into the fat layer while avoiding the underlying muscle tissue, which would cause the drug to absorb much faster than intended.
A commonly used method for determining the appropriate angle is the “skin pinch” test. If the individual can pinch up a fold of skin and fatty tissue measuring two inches (about 5 centimeters) between their thumb and forefinger, a 90-degree angle is safe for needle insertion. Inserting the needle straight in at 90 degrees minimizes tissue trauma and is often less painful.
If only one inch (about 2.5 centimeters) of tissue can be pinched up, or if the individual has very little body fat, the needle should be inserted at a 45-degree angle. This shallower angle ensures the needle does not reach the muscle layer, which is a greater risk in thinner individuals. The length of the needle is also a factor, as shorter needles may permit a 90-degree insertion even without a large skin fold.
Choosing Safe Injection Locations
Subcutaneous injections must be administered into areas of the body that have a sufficient layer of fatty tissue. The most suitable sites include the abdomen, the front and outer sides of the thighs, and the back of the upper arms. These locations offer easy access and a consistent layer of subcutaneous fat.
When using the abdomen, injections should be placed below the ribs and above the hip bones, keeping at least two inches (5 centimeters) away from the navel. The upper arm site is recommended if another person is administering the injection. The thigh area is suitable in the section between the knee and the hip.
Avoid injecting into areas that are bruised, swollen, tender, or show signs of skin damage or scarring. Rotating the injection site is necessary to maintain skin health and ensure consistent medication absorption. Injecting repeatedly into the same spot can lead to the formation of a fatty lump called lipohypertrophy, which can interfere with drug absorption. Each new injection should be placed at least one inch away from the previous site.
The Subcutaneous Injection Procedure (Step-by-Step)
The injection process begins with meticulous hand hygiene, washing with soap and running water. All necessary supplies must be gathered, including the medication, syringe, alcohol wipes, and a puncture-proof sharps container. The medication should be prepared according to instructions, ensuring the correct dose is drawn and any air bubbles are removed from the syringe.
After selecting the appropriate site, the area must be cleaned with an antiseptic swab, wiping in a circular motion beginning at the center and moving outward. Allow the skin to air dry completely before proceeding, as this prevents stinging during the injection. The skin fold is created by gently pinching the fatty tissue between the thumb and forefinger of the non-dominant hand, if necessary.
The needle is then inserted quickly and smoothly into the pinched skin fold at the determined 45- or 90-degree angle. A rapid motion is less uncomfortable than a slow insertion. Once the needle is fully inserted, the skin fold can be released.
The medication is administered by pushing the plunger down slowly and steadily until the syringe is empty. A slow injection helps minimize discomfort at the site. After the full dose is delivered, the needle is quickly removed from the skin at the same angle it was inserted. Pressure can be applied to the site with a clean piece of gauze or a cotton ball, but the area should not be rubbed, as rubbing can increase the risk of bruising or tissue damage. The used syringe and needle must be immediately placed into the sharps container without attempting to recap the needle.