Additional blood tests are indicated depending on the presenting GI symptoms and differential diagnoses as outlined in the algorithm. They potentially include ESR, CRP, red cell folate, iron studies, vitamin B12, thyroid function test, coeliac serology (TTG IgA), magnesium
Elevated ESR/CRP	→	Consider the following possibilities: ▸ Infection (including SIBO). ▸ Inflammation (including IBD). ▸ Recurrent malignancy. ▸ Non-GI causes (eg, rheumatoid arthritis, vasculitis, connective tissue disorders).
RBC folate deficiency	→	▸ Consider referral to dietitian for specialist dietetic advice/supplementation.
Iron deficiency: ferritin, % transferrin saturation, red cell indices	→	▸ If iron is low, discuss with supervising clinician and oncology team within 2 weeks. ▸ If intolerant of oral iron: consider IV iron infusion.
If excess iron	→	▸ Consider haemochromatosis: discuss with supervising clinician and consider genetic testing.
Low vitamin B12	→	▸ Exclude possibility of inadequate dietary intake—if this is the probable cause, consider trial of oral vitamin B12 supplements. ▸ Consider possibility of pernicious anaemia—check parietal cell antibody. ▸ Exclude SIBO (page 17). Recheck result after treatment with antibiotics. ▸ If confirmed on repeat testing and not treatable with oral replacement, ask GP to arrange lifelong intramuscular replacement.
Abnormal thyroid function tests	→	▸ If TSH suppressed (<0.5 mIU/L), recheck result with thyroid autoantibodies. ▸ If TSH suppression confirmed, request GP to organise/refer for radiological imaging and treatment.
Abnormal thyroid function tests	→	▸ If TSH elevated (>4.0 mIU/L). Re-check result. Also check morning cortisol if Na ≤135 mmol/L and K >4 mmol/L or raised urea or creatinine. ▸ If TSH elevation confirmed: start thyroid replacement medication. Request GP monitor long term. Review bowel function after 6–8 weeks.
Abnormal coeliac serology	→	▸ If IgA deficient, request IgG coeliac screen. ▸ If TTG elevated, confirm with duodenal biopsy. ▸ Refer for specialist dietetic advice.
Serum Mg2+	→	▸ If <0.3 mmol/L: *this is an emergency*. ▸ If 0.3–0.5, consider IV replacement if symptomatic or fall in Mg2 level has been acute. If oral replacement is given, check for response after 5–7 days with repeat blood tests. ▸ If oral replacement is used, Mg oxide or Mg aspartate provide better bioavailability and cause less diarrhoea than other Mg preparations.

CRP, C-reactive protein; ESR, erythrocyte sedimentation rate; GI, gastrointestinal; GP, general practitioner; IBD, inflammatory bowel disease; IgA, immunoglobulin A; IgG, immunoglobulin G; RBC, red blood cell; SIBO, small intestinal bacterial overgrowth; TSH, thyroid stimulating hormone; TTG, tissue transglutaminase.