BLOOD TESTS: WHAT, WHEN AND WHY? Fiona Clarke

Blood Tests: Why, What and When? • Why do we take blood? • • • • • • •

To make a diagnosis To rule out a diagnosis To assess disease activity/severity Screening for other health issues To predict prognosis To monitor disease activity and effectiveness of treatment To monitor for side effects of treatment

Making/ruling out a diagnosis • Inflammatory markers • Autoantibodies • Biochemistry • Genetic

Inflammatory markers • ESR, CRP • • • • •

Similar causes Infection Inflammation/disease activity Malignancy Obesity

Significance • Very high (ESR >age plus ten divided by 2 for women, age divided by 2 for men) • Comparison to what is normal for the patient • CRP quicker to respond • N.B. SLE

Full Blood Count • WBC • Hb • RBC • HCT • MCV • MCH • RDW • PLT

• Neutrophil Count • Lymphocyte Count • Monocyte Count • Eosinophil Count • Basophil Count • HYPOCHROMASIA

WBC • High • • • • •

Infection Steroids Inflammation Malignancy Trauma

• Low • • • • • •

Drugs Infections Hypersplenism Antibodies Marrow failure B12/folate deficiency

Anaemia • Microcytic • Iron deficiency (dietary, blood loss)

• Normocytic • Chronic disease • Marrow failure • Haemolysis, hypothyroidism, pregnancy

• Macrocytic • • • •

B12 or folate deficiency Hypothyroidism Alcohol/liver disease Haemolysis (reticulocytosis)

• Often mixed picture in Rheumatoid/CTD

Polycythaemia • Primary • Polycythaemia Rubra Vera

• Secondary • • • •

Dehydration COPD Tumours Altitude

Platelets • Thrombocytopoenia • Thrombocytosis

Autoantibodies • Rheumtoid factor • Sensitivity 28% and specificity 87%

• Anti-cyclic citrullinated peptide antibody • Sensitivity between 69.6% and 77.5% and specificity between 87.8% and 96.4%.

• Anti nuclear factor, Ro, La, anti Sm, RNP, Jo-1, anti scl 70 • Anticardiolipin, ß2 glycoprotein, lupus anticoagulant • Gastric parietal cell, anti-mitochondrial, anti smooth muscle

Sensitivity and specificity • Sensitivity • Sensitivity refers to the test's ability to correctly detect patients who do have the condition. Consider the example of a medical test used to identify a disease. The sensitivity of the test is the proportion of people who test positive for the disease among those who have the disease. • A negative result in a test with high sensitivity is useful for ruling out disease. A high sensitivity test is reliable when its result is negative, since it rarely misdiagnoses those who have the disease. A test with 100% sensitivity will recognize all patients with the disease by testing positive. A negative test result would definitively rule out presence of the disease in a patient.

Sensitivity and specificity • Specificity • Specificity relates to the test's ability to correctly detect patients without a condition. Consider the example of a medical test for diagnosing a disease. Specificity of a test is the proportion of healthy patients known not to have the disease, who will test negative for it. • A positive result in a test with high specificity is useful for ruling in disease. The test rarely gives positive results in healthy patients. A test with 100% specificity will read negative, and accurately exclude disease from all healthy patients. A positive result signifies a high probability of the presence of disease. • A negative result in a test with high specificity is not useful for ruling out disease.

Biochemistry • Urea & Electrolytes • • • • • •

Sodium Potassium Bicarbonate Urea Creatinine eGFR

Biochemistry • Liver Function tests • • • • • • •

Albumin Total Protein Total Bilirubin ALT ALP GGT Globulin

Biochemistry • Bone Chemistry • • • •

Calcium Corrected Calcium Vit D PTH

• Urate

Prognosis • Inflammatory markers • Autoantibody titres

Disease activity • CRP, ESR • Haemoglobin, Liver function, ferritin

DMARD monitoring • Abnormalities •

may reflect damage/potential damage caused by the drug

•

May be an unimportant effect of the drug or have little clinical significance

Important mesages • Most of our patients will have results that fall outside of the normal limits for the lab • Some of these are not significant and it is important to differentiate • Always compare to previous results for that patient • If you are not sure, ask!