Lump in the armpit: causes, red flags and when to get it checked

The armpit (axilla) holds 20 to 40 lymph nodes that filter fluid from the arm, breast and upper trunk. They swell when fighting infection or inflammation. Other lumps come from skin and sweat glands, fat or, less often, from disease in the nodes themselves.

• Reactive lymph node: after an infection of the hand, arm or breast, a cut, shaving irritation or vaccination.

• Skin causes: boil or abscess, infected hair follicle, hidradenitis suppurativa, sebaceous cyst or lipoma.

• Allergic or irritant reaction to deodorant or shaving.

• Vaccination reaction: nodes on the vaccinated arm side can swell for some weeks.

• Breast conditions: accessory breast tissue and, uncommonly, breast cancer spreading to nodes.

• Lymphoma, leukaemia or other blood cancers, with painless rubbery nodes.

• Infections such as cat scratch disease, tuberculosis and HIV.

• Autoimmune diseases (lupus, rheumatoid arthritis, sarcoidosis) and metastatic cancer (melanoma, lung, others).

A lump that is hard, fixed or growing, larger than about 2 cm, lasting beyond four weeks, with weight loss, night sweats, fevers, itching, fatigue, a breast lump, nipple or skin change, other enlarged glands, or a history of cancer needs prompt examination.

• Same-day GP: a hot, red, very painful lump with fever (abscess).

• Urgent GP referral: a persistent hard lump, or a lump with a breast abnormality, weight loss or night sweats.

• GP within weeks: a lump lasting more than two to three weeks.

• Routine: a small soft tender node after a cut or infection that is shrinking.

The doctor asks how long it has been present, size changes, pain, skin problems, shaving and deodorant use, vaccinations, infections, weight loss, sweats, breast symptoms and personal or family history of cancer. They examine the armpit, both breasts, other node areas, skin and abdomen.

• Blood tests: full blood count, inflammatory markers and infection screens.

• Ultrasound of the armpit and breast: the first imaging test to assess a lump.

• Mammogram: for women at appropriate age or with suspicious findings.

• Needle biopsy or lymph node biopsy: for persisting or suspicious nodes.

• CT scan: for staging if lymphoma or cancer is found.

Avoid squeezing the lump, keep the area clean, hold off shaving and deodorant if irritated, and apply a warm compress to a boil. Note the size and date, and recheck in two to three weeks. Do not ignore a lump that is not shrinking.

The NHS route is a GP, with rapid breast clinic or haematology referral for suspicious lumps. Private breast clinics and haematology consultations offer one-stop imaging and biopsy.