Understanding Blood Tests: What They Detect and When to Repeat

What can blood tests detect?

Blood tests are one of the fastest ways to get objective information about how your body is functioning. Depending on the panel, they can detect or help screen for:

• Infections and inflammation (e.g., elevated white cells, CRP).
• Red blood cell problems and anemia (haemoglobin, haematocrit, MCV, RDW).
• Cholesterol and cardiovascular risk (total cholesterol, LDL, HDL, triglycerides, Chol:HDL ratios).
• Diabetes and blood sugar control (HbA1c).
• Liver and kidney function (ALT, ALP, GGT, bilirubin, creatinine, eGFR, urea).
• Thyroid dysfunction (TSH, FT3, FT4).
• Iron status and storage (serum iron, ferritin).
• Vitamin and mineral deficiencies (vitamin D, B12, folate, magnesium).
• Hormonal imbalances (testosterone, oestradiol, FSH, LH, cortisol, progesterone).
• Muscle damage or stress (CK).
• Markers that raise concern for chronic disease risk, including some cancer-associated signals (depending on markers included).

Why might you need a repeat blood test?

Repeating a blood test is common and often necessary. Typical reasons include:

• Confirming an unexpected or borderline result,labs sometimes need verification.
• Monitoring treatment effectiveness (e.g., cholesterol-lowering medication, diabetes management, thyroid therapy).
• Tracking progress of a diagnosed condition or recovery (e.g., improving iron stores or liver enzymes).
• Timing-related variations,some tests depend on fasting, time of day, or menstrual cycle.
• Lab variability — small differences between labs or methods can require repeat testing.
• Medication changes or new symptoms that require re-evaluation.

When a clinician asks for a repeat test, it’s usually to ensure accuracy or to follow up on something that may affect your care plan.

Common blood tests (a handy list)

Here’s a practical, categorized list of common blood tests you’ll see on lab forms or discussed by clinicians:

• Full Blood Count (FBC or CBC): haemoglobin, haematocrit, red/white blood cells, platelets, MCV, MCH, MCHC, RDW.
• Lipid profile: total cholesterol, HDL, LDL, triglycerides, Chol:HDL ratio, Non-HDL cholesterol.
• Diabetes: HbA1c (glycated haemoglobin), fasting glucose (when ordered).
• Liver function tests (LFTs): ALT, AST, ALP, GGT, total bilirubin, albumin, total protein.
• Kidney function: creatinine, eGFR, urea.
• Inflammation: CRP or hsCRP.
• Iron studies: ferritin, serum iron, transferrin (or TIBC when ordered).
• Thyroid: TSH, FT4, FT3.
• Vitamins/minerals: vitamin D, active B12, serum folate, magnesium.
• Hormones: testosterone, free testosterone (calculated), SHBG, FSH, LH, oestradiol, progesterone, cortisol.
• Others: CK (muscle), uric acid, PSA (prostate-specific antigen) when indicated.

Understanding blood test abbreviations

Laboratory reports use many acronyms. Here are common ones and what they mean:

• FBC or CBC,Full Blood Count / Complete Blood Count: basic blood cell profile.
• Hb or Hgb,Haemoglobin: oxygen-carrying protein in red cells.
• MCV — Mean Cell Volume: average size of red blood cells.
• MCH/MCHC,Mean Cell Hb / Mean Cell Hb Concentration: measures of haemoglobin content.
• ALT / AST,Liver enzymes indicating liver cell injury.
• ALP — Alkaline Phosphatase: linked to liver and bone health.
• GGT,Gamma-Glutamyl Transferase: liver and bile duct marker.
• eGFR,Estimated Glomerular Filtration Rate: kidney function estimate.
• hsCRP — High-sensitivity C-Reactive Protein: sensitive inflammation marker tied to heart risk.
• HbA1c,Glycated Haemoglobin: three-month average blood sugar.
• HDL / LDL / TG,Types of cholesterol: good, bad, and triglycerides.
• TSH / FT3 / FT4 — Thyroid-stimulating hormone and active thyroid hormones.
• CK,Creatine Kinase: muscle enzyme elevated with muscle injury.
• PSA,Prostate-Specific Antigen: screening marker for prostate disease (when ordered).

Why a bruise after a blood test might be getting worse — and what to do

Minor bruising at the needle site is common. It happens when blood leaks from the punctured vessel into surrounding tissue. Usually it fades over 1–2 weeks. Bruising that worsens or is unusually large may be caused by:

• Blood-thinning medications (warfarin, direct oral anticoagulants, aspirin, some NSAIDs).
• Underlying bleeding or clotting disorders.
• Fragile veins, multiple attempts at drawing blood, or movement during the draw.
• Inadequate pressure applied after removing the needle.

First aid and when to seek help:

• Apply firm pressure for 5–10 minutes immediately after the draw to reduce bleeding into the tissue.
• Use a cold pack for 10–15 minutes initially to limit swelling, then warm compresses after 24 hours to help reabsorb the blood.
• Elevate the arm and avoid strenuous use of that arm for 24 hours.
• Avoid NSAIDs like ibuprofen if you are concerned about bleeding unless advised by your clinician.
• Contact a healthcare professional if the bruise keeps enlarging, is very painful, the area becomes red/warm (possible infection), you have numbness, or you notice unusual bleeding elsewhere.

Choosing the right panel for a full view

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Final tips

• Discuss abnormal or borderline results with your clinician — context and symptoms matter.
• Follow fasting or preparation instructions closely to avoid misleading results.
• Keep a record of your results over time to spot trends that one-off tests can miss.