Chapter 13: Diagnostic tests
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Source: Adapted from NHS Pathology ([156]), Pagana and Pagana ([177]), WHO ([260]).
Evidence‐based approaches
Rationale
Diagnostic tests are essential in the healthcare setting but their selection and use must be considered carefully. The overuse of or inefficient use of diagnostic tests is a contributor to needless healthcare costs. This leads to poor‐quality services and continuing financial pressure on departments and organizations (Korenstein et al. [118], Pagana and Pagana [177]). Hence, it is essential that healthcare providers and professionals ensure that the tests used are of adequate benefit to the patient's health (Pagana and Pagana [177], Qaseem et al. [197]). For example, a faecal sample from a patient who has a history of recent travel would be investigated for organisms not normally looked for in a specimen from a patient without such a history. The sensitivity of the organism to a range of antimicrobials could then be tested to decide on the most appropriate and effective mode of treatment (Higgins [84]).
Successful laboratory diagnosis depends upon the collection of specimens at the appropriate time, using the correct technique and equipment, and transporting them to the designated laboratory safely and without delay (Box 13.1). For this to be achieved, it is essential that there is good liaison between medical, nursing, portering and laboratory staff. Diagnostic tests should not have to be repeated due to inadequate patient preparation, procedures or specimen collection techniques (Pagana and Pagana [177]).
Indications
Conducting a diagnostic test is often the first crucial step in determining a diagnosis and subsequent mode of treatment for patients. In other contexts, the collection or diagnostic test may help to determine variation from normal values, such as in blood sampling or endoscopic findings.
Box 13.1
Good practice in specimen collection
- Appropriate to the patient's clinical presentation
- Collected at the right time
- Collected in a way that minimizes the risk of contamination
- Collected in a manner that minimizes the health and safety risk to all staff handling the sample
- Collected using the correct technique, with the correct equipment and in the correct container
- Documented clearly, informatively and accurately on the request forms
- Stored and transported appropriately
Principles of care
Nursing staff play a key role within the diagnostic testing process because they often identify the need for diagnostic tests, initiate the collection of specimens, and assume responsibility for timely and safe transportation to the laboratory (Higgins [84], Keogh [114]). The interpretation of findings or results is also a key role for nurses in various settings and facilitates timely and collaborative treatment plans (Pagana and Pagana [177]). Specimen collection is often the first crucial step in investigations that define the nature of the disease, determine a diagnosis and therefore decide the mode of treatment. Patient education is also a crucial component in ensuring accuracy and success of diagnostic tests. Patient understanding of all steps before, during and after the test or investigation is necessary and should form part of the nursing process (Pagana and Pagana [177]).
Methods of investigation
Various healthcare professionals, such as doctors, nurses, pharmacists and allied healthcare professionals, are able to identify the need for and request a variety of diagnostic tests. Those requesting a diagnostic test must also be able to interpret the findings. Each professional must have the ability to clinically assess the patient, and this assessment then informs the choice of diagnostic test. An understanding of the laboratory methods available is also important for those requesting a diagnostic test.
Initial examination and assessment
The initial examination and assessment of the patient will determine the potential diagnostic tests or samples that are required. Depending on how the patient progresses, their clinical history, a physical assessment and/or their symptom progression will subsequently determine the need for further diagnostic tests. The results of these tests will then form a treatment plan. The initial assessment is often undertaken by nurses and other healthcare professionals (Keogh [114], Pagana and Pagana [177]).
Cytology
Cytology is the study of the cell, its structure, structural transformations, molecular biology and cell physiology. The specimen can be a fine‐needle aspiration, a sample of body fluid or a scrape/brush. The specimen is placed onto a glass slide and examined for the presence of abnormal cells. These include benign, pre‐cancerous and cancer cells. The test can also be used to diagnose infective processes (Chernecky and Berger [32]).
Direct microscopy
The majority of specimens will then undergo direct microscopic investigation, which is valuable as an early indication of the causative organism. High magnification is required to visualize viruses, which are then identified according to their characteristic shapes (Gould and Brooker [75]). Certain parasitic protozoa, such as those causing malaria, are identified by direct microscopy, which necessitates the specimen being delivered to the laboratory as quickly as possible, while the protozoa are mobile and therefore visible (Higgins [84]). In combination with clinical presentations, this may be enough to initiate or change targeted treatments until a more definitive diagnosis is reached (Weston [255]).
Gram staining
Gram staining is a process for identifying bacterial species. A staining substance is added to a sample to differentiate the types of organism present. Cells with differing properties stain differently in relation to the structure of their cell wall. Gram staining allows for the differentiation of gram‐positive bacteria (e.g. Staphylococcus aureus), which stain purple, and gram‐negative bacteria (e.g. Escherichia coli), which stain pink when viewed under the microscope. The findings can be used to guide the choice of antimicrobial therapy until other investigation methods can provide a definitive identification of pathogenic micro‐organisms. In blood cultures, gram staining is 67.9% sensitive for the detection of bacteria (Chernecky and Berger [32]).
Histology
Histology is the study of cells and tissues within the body. It also studies how the tissues are arranged to form organs. The histological focus is on the structure of individual cells and how they are arranged to form the individual organs. The types of tissues that are recognized are epithelial, connective, muscular and nervous (Kierszenbaum and Tres [116], Mescher [142]).
The tissues are examined under a light microscope, where light passes through the tissue components after they have been stained. As most tissues are colourless, they are stained with dyes to enable visualization. An alternative is the electron microscope, in which cells and tissue can be viewed at magnifications of about 120,000 times (Kierszenbaum and Tres [116], Mescher [142]).
Serology
A serological test looks for antibodies in a patient's blood. Serology is useful in identifying viral infections and bacterial infections with difficult culture organisms and is based upon the patient's immunological reaction. Detection of antigens or antibodies in serum, which are activated in response to infection, may suggest that the patient is, or has been, infected (Higgins [84], Keogh [114]).
