What should you know about a hard blue lump under the skin?

Blue or purple colour may reflect blood beneath the skin, a blood-vessel-related change or a pigmented lesion. A tender, warm lump can occur with an inflamed blockage or boil, while a firm, slowly changing nodule may have another cause. A skin lump is not diagnosed by colour alone, and most explanations cannot be separated reliably without looking and feeling the area.

Basal cell carcinoma can sometimes appear as a persistent skin growth or sore, although a hard blue lump is not specific to it. Note when the lump appeared, whether it followed an injury or injection, and whether it bleeds, crusts, drains or changes size.

Arrange a clinician or dermatology assessment for a lump that persists, grows, repeatedly bleeds or remains unexplained. Photograph it beside a familiar object so change can be noticed, but do not squeeze, lance or repeatedly press it. Keep the surface clean and avoid irritating products. A clinician may decide that observation, imaging, a sample or treatment is appropriate after examination.

Seek urgent care for rapidly increasing swelling, severe pain, spreading redness, fever, heavy bleeding, faintness or a limb that becomes cold, numb or weak. Arrange prompt medical review for a new darkening lesion, a non-healing surface, repeated crusting or a lump that becomes fixed. If it appeared after an injection and is associated with severe pain or unusual skin colour, contact the treating service promptly.

Ask what features make the lump more consistent with a bruise, boil, vessel or growth; whether it needs dermoscopy or sampling; and whether a steroid injection into the skin would be suitable or should be avoided.