IM, SubQ & Intradermal Injection

Three parenteral routes share a syringe but differ in angle, depth, site selection, and volume limits. Intramuscular (IM) injections are given at a 90-degree angle into muscle tissue, reaching the highly vascular muscle layer for rapid absorption. The ventrogluteal site is preferred for adults — it avoids the sciatic nerve and has the thickest muscle mass. The vastus lateralis is preferred for infants. The deltoid works for volumes ≤1 mL (adults). IM volume limits: up to 3 mL in large muscles (ventrogluteal), 1 mL in the deltoid. Aspiration is no longer recommended for IM injections per CDC guidelines. Subcutaneous (SubQ) injections target the adipose layer at a 45- to 90-degree angle (45 degrees is standard; 90 degrees may be used with adequate pinchable tissue or short needles), with volumes up to 1 mL. Common sites: abdomen (insulin), posterior upper arm, anterior thigh. Rotate sites systematically to prevent lipohypertrophy. Intradermal (ID) injections go into the dermis at a 5- to 15-degree angle, bevel up, forming a visible wheal (6–10 mm bleb). Volume is tiny — 0.01 to 0.1 mL. The inner forearm is the standard site. TB skin tests (Mantoux) are the classic example — if no wheal forms, the injection went too deep and must be repeated.