Introduction to Diagnostic Testing

Introduction to Diagnostic Testing Dr Nick Hopcroft Learning Outcomes: Describe the selection of investigations used for the differential diagnosis of common and/or important conditions: Explain appropriate use of the diagnostic laboratory Outline the approach to selection of appropriate la...

Introduction to Diagnostic Testing Dr Nick Hopcroft Learning Outcomes: Describe the selection of investigations used for the differential diagnosis of common and/or important conditions: Explain appropriate use of the diagnostic laboratory Outline the approach to selection of appropriate laboratory investigations Outline the uses of laboratory investigations in the diagnosis of disease and management of patients, including biochemistry, microbiology, haematology and histopathology investigations Interpret the results of routine laboratory investigations in biochemistry, microbiology, haematology and histopathology  Diagnostic Process History (presenting complaint, past medical history, social history, ICE) ↓ Examination (general, systems) ↓ Differential Diagnosis ↓ Investigations (biochemistry, microbiology, haematology, histopathology) / Imaging ↓ Diagnosis ↓ Management Uses of Investigations Diagnosi s Suitabilit Screenin y for g treatme nt Investigati ons Monitori ng of Prognosi treatme s nt  Appropriate Use of Investigations Must be relevant, accurate and used within the clinical context Irrelevant investigations lead to unnecessary risk, cost and false positive results Errors may occur before, during and after testing Clinical context is vital and must be both provided with requests and considered when interpreting results  Appropriate Use of Investigations Should be requested after you have clinically assessed the patient The information you gain must outweigh the risk to the patient Investigations should start with the least invasive and least risky and move up e.g. Urine Blood CT Sampl Test Scan e  Biochemistry: Liver Blood Tests Many blood proteins are synthesised in the liver, so a blood sample can be taken and tested for various protein levels to assess liver function Theses tests can give information about liver disease, the involvement of the liver in other disease and the effects of medication They can be used to distinguish between different disorders of the liver and to assess the extent of damage Some tests give a measure of liver functionality whilst others reflect the structural integrity of liver cells  Biochemistry: Liver Blood Tests Albumin – major component of total blood protein, made specifically by the liver; decreased in chronic liver disease Aminotransferases – found in the liver and elsewhere; very high levels in the blood usually indicate liver damage Bilirubin – breakdown product of heme, usually cleared by the liver; elevated levels suggest liver disease Alkaline phosphatase – found in the liver and elsewhere; blood levels may be increased in acute viral hepatitis Gamma glutamyl transferase – found in the liver and elsewhere; blood levels may be increased in hepatitis and chronic liver disease  Biochemistry: Urea and Electrolytes Sodium – major contributor to extracellular fluid (ECF) osmolality, which influences water balance Potassium – major intracellular cation, raised values seen in renal impairment Urea – by-product of the deamination of amino acids and used to indicate kidney function Creatinine – by-product of muscle breakdown and used to indicate kidney function eGFR – estimated glomerular filtration rate, calculated using the creatinine measurement and other factors Haematology: Full Blood Count Haemoglobin (Hb) – concentration of haemoglobin in the blood; lowered in anaemia Haematocrit / PCV – packed cell volume, proportion of blood volume made up of cells Red cell count (RCC) – estimated number of red blood cells; can differentiate between different types of anaemia Mean corpuscular volume (MCV) – average volume of the red blood cells; changes of size in either direction can differentiate between different types of anaemia Mean corpuscular haemoglobin (MCH) / Mean corpuscular haemoglobin concentration (MCHC) White blood cells (WBC) / Platelets  Histopathology The microscopic examination of tissue from biopsy, surgery or autopsy for signs of disease Samples are cut, fixed and stained before viewing – the most common stain is hematoxylin and eosin (H&E) Histopathology can detect cellular changes such as metaplasia and neoplasia It can also detect an increased presence of cells not normally found in the tissue in high levels (e.g. cells associated with inflammation)  Microbiology Samples : Swabs = pus, skin, nose, throat, urethra, vagina etc. Body fluids = pus, urine, faeces, blood, CSF etc. Body tissues = biopsies (rarely)  Microscopy, Culture and Sensitivities Usually relates to bacteria or fungi Microscopy uses specific stains (e.g. Gram) Culture media are selective for certain bacteria Antibiotic sensitivity testing done from discs on culture Biochemical profiling of isolates gives more precise  Antigen Detection Tests An antigen is a biological molecule able to cause an immune response – i.e. antibodies will be produced Some antigens are highly specific to a particular pathogen, so detection of these antigens can make a good diagnostic test Antibodies can be produced against these antigens and are used in antigen detection tests for that pathogen These tests are useful for rapid and early detection of pathogens, especially when other techniques not feasible Antigen Detection Tests Antigen Detection Tests  Antigen Detection Tests Negative Non-Falciparum Falciparum or Mixed  Polymerase Chain Reaction (PCR) PCR is a way of exponentially replicating a specific DNA sequence, making small quantities detectable Some DNA sequences are highly specific to a particular pathogen, so detection of these sequences can make a good diagnostic test ‘Primers’ are produced that are complementary to these sequences and are used in PCR tests for that pathogen These tests are useful for rapid and early detection of pathogens, especially when small numbers of organisms are present or when other techniques not feasible Polymerase Chain Reaction (PCR) Polymerase Chain Reaction (PCR) Polymerase Chain Reaction (PCR) Polymerase Chain Reaction (PCR) Polymerase Chain Reaction (PCR)  Reference Ranges Normal values vary across the population with a normal distribution, so comparing a test result to the average (mean) is of limited value 95% of normal values lie within 2 standard deviations of the mean, so this is usually taken as the normal  Conclusions Only order tests when clinically required Understand the reference range to which the result is being compared Interpret the results in the context of the clinical information Understand that there are limitations in test results Acknowledgement to Professor Mark Bailey [email protected]