Specimen Integrity & Lab Test Parameters

Specimen Integrity and Test Results

• Specimen integrity is critical for quality and accurate lab testing.
• Hemolysis, storage times, temperature, transport methods, interfering blood conditions (icterus, lipemia, turbidity), inadequate blood volume with additives, and incorrect collection tubes can all affect test results.
• A hemolyzed specimen yields falsely elevated results for potassium, magnesium, calcium, and iron.
• Maintenance of specimen integrity for microbiologic samples is discussed in Chapter 8.
• Chapter 8 covers specimen integrity for arterial blood gas samples.

Laboratory Test Parameters

• Quality test results must be accurate, precise, and clinically useful for diagnosis and monitoring.
• Usefulness is assessed by predictive value, including clinical sensitivity, specificity, positive predictive value, and negative predictive value.
• A test is considered:
• True positive (TP) if positive in a patient with a disease.
• False negative (FN) if negative in a patient with the disease.
• True negative (TN) if negative in a disease-free patient.
• False positive (FP) if positive in a disease-free patient.
• TP, FN, TN, and FP are grouped into statistical parameters that describe test result accuracy in relation to a particular disease.
• Sensitivity is the frequency of positive test results in patients with a disease.
• A sensitivity of 98% indicates that 98% of patients with the disease will be detected by the test (TP), with 2% being negative (FN).
• If a patient has the disease, expect a positive test.
• A negative result on a highly sensitive test confidently rules out the disorder.
• Specificity represents the frequency of negative test results in patients without the disease.
• A specificity of 98% indicates that 98% of disease-free patients will test negative (TN), with 2% testing positive (FP).
• If a patient does not have the disease, expect a negative test.
• A positive result on a highly specific test can confidently rule in the disorder.
• Laboratory test results interpretations are based on comparison to a reference interval or range.
• Reference intervals depend on factors such as patient age, gender, sample population, and test method.
• Laboratories should have specific reference intervals for their facility or population, calculated to include 95% of a healthy population.
• Reference intervals represent typical values, caution against using "normal values" due to potential misleading nature since a reference interval does not mean the patient does or does not have abnormal conditions.
• Comparison with related test results, along with lab findings, should be taken into account and correlated for appropriateness, diagnosis, and/or treatment.

Hematology laboratory tests

• Clinical laboratory tests in hematology are divided into:
• General hematology tests for normal and abnormal blood cells
• Coagulation tests for evaluating blood clotting

Complete Blood Count

• Includes the complete blood count (CBC), white blood cell differential count, reticulocyte count, and erythrocyte sedimentation rate

Routine coagulation tests

• Prothrombin time, international normalized ratio (INR), and activated partial thromboplastin time (APTT) assess wellness status, detect conditions, and monitor therapies.

Complete Blood Count (CBC)

• CBC assesses quantity and appearance of blood cells (RBCs), WBCs, and platelets
• CBC includes:
• WBC count
• WBC differential count
• RBC count
• Hemoglobin (Hct)
• RBC indices
• Platelet count
• Reticulocyte count
• Table 7.1 contains sample adult intervals for CBC components in common standard international (SI) units.
• Discussion focuses on WBC differential count, RBCs, and coagulation studies, in that order.

White Blood Cells

• WBCs function as part of the immune system, defending the body from pathogenic microorganisms and antigens.
• WBCs develop in the bone marrow from lymphoid tissue, influenced by growth factors.
• Mature WBCs are released into the peripheral blood, including segmented neutrophils (polys), eosinophils, basophils, lymphocytes, and monocytes.
• White Blood Cell Differential Count (CBC) determines the total number of WBCs per microliter of blood and performs an automated differential count.
• Segmented neutrophils, eosinophils, and basophils are granulocytes, with differing functions.
• Segmented neutrophils (polys/nuclear leukocytes) constitute most of the circulating WBCs in peripheral blood.
• A band is a slightly less mature neutrophil where the nucleus has not yet segmented.
• Bands are included in the segmented neutrophil count.
• Percentage and absolute number are usually determined.
• If the instrument flags suspected abnormalities, a slide review and differential count are performed scientist to see if there are morphological features that may be the presence of immature cells
• The relative count is the percentage of each cell type among all WBCs counted.
• Each cell type is determined by multiplying its percentage or relative count to the total WBC count.
• Table 7.2 provides sample relative and absolute counts.
• Because the relative count is derived, increases one cell type will decrease the other WBC percentages
• The absolute count provides a more useful assessment. Comparing intervals in Table 7.2, confirms an absolute increase in neutrophils.
• For example, suppose an adult patient with a total WBC count of 5 x 10^9/L has a relative neutrophil count of 85%. The absolute neutrophil counts are 17 x 10^9/L for lymphocytes.
• Count is within the reference with the relative count for the lymphocytes.

Specimen integrity dramatically influences lab testing accuracy. Factors like hemolysis, storage conditions, and collection methods can alter results. Accurate test results hinge on precision and clinical utility, evaluated through predictive values like sensitivity and specificity.