Definitions of cd4 count

Laboratories Glossary

analyte.
An analyte is the substance that is being identified or measured in a lab test.

CD4.
Also known as T4 cells, CD4 cells are one of several types of T cells that are important to the immune response. They protect against viral, fungal, and protozoal infections and are the cells most susceptible to HIV. A CD4 count is an indicator of the health of patients’ immune systems and thus their risk of developing opportunistic infection. Test results from a CD4 count can also be used to judge when antiretroviral therapy should begin (see T cell count).

coefficient of variation.
The coefficient of variation (CV) is a measurement of the precision (or reproducibility) of a laboratory test or process. Modern instruments have a CV of 3 percent to 5 percent. When all other parameters are equal, the lower the CV, the better the test.

consumables.
Consumables are items that are used once in performing a test and are not reused; for example, microscope slides and cover slips.

durables.
Durables are items that can be reused for multiple tests; for example, some types of glassware that can be sterilized and reused.

ELISA test.
The enzyme-linked immunosorbent assay (ELISA) is used to detect the presence of antibodies in serum. ELISA is used for first-line screening for HIV antibodies; a positive result indicates that antibodies have been detected. The test is sensitive but not specific; and thus a positive ELISA is typically confirmed with a Western blot assay. In resource-constrained settings, first-line screening can be done with either a rapid assay and confirmatory ELISA or a combination of rapid assays

equipment.
Instruments used in a laboratory to conduct a test are considered equipment. These items often are automated and require regularly scheduled maintenance; examples include microscopes and hematology machines. (See below for descriptions of the different systems commonly found in laboratories.)

external quality assurance.
External quality assurance (EQA) is a program that allows testing sites to assess the quality of their performance by comparing their results with those of other laboratories. This is done by analyzing proficiency panels or blind rechecking. EQA often includes on-site evaluation of the laboratory to review the quality of test performance and operations.

feasibility.
The feasibility of providing a specific test depends on the availability of all the elements needed to conduct the test: proper equipment; standard operating procedures; adequate quality of water and reagents; and a clean, constant power supply. The human resources are equally necessary and include adequate staffing, training, and supervision.

good laboratory practice.
Good laboratory practice includes the practices, processes, and conditions required for high-quality laboratory studies to be planned, performed, monitored, and reported.

maintenance spares.
Maintenance spares are often overlooked but are necessary for a functioning laboratory. Without adequate spare parts, the laboratory cannot provide reliable service. Most manufacturers can provide an accurate prediction of the type and number of parts required for a given instrument for one year.

medical technologist.
A medical technologist—or clinical laboratory scientist—typically has a baccalaureate degree and a specialized internship. Many nations require certification examinations and continuing education. Some states also require licensure.

medical technician.
A medical technician typically has a two-year specialized education and is supervised by a medical technologist.

open systems.
Open systems are laboratory instruments that do not require a specific brand of reagents. Open systems do not rely on a single source but can use reagents from any manufacturer that develops the specifications needed for the test.

pathologist.
A pathologist is a medical doctor specializing in disease or pathology.

preventative maintenance.
Preventative maintenance is the sum of the tasks performed on equipment, based on the manufacturer’s schedule, to prevent failure of an instrument. It is a proactive process designed to prevent testing errors from instrument failure; it is part of the quality assurance process.

quality assurance.
Quality assurance (QA) manages the quality of all aspects of the testing process. QA considers pre-analytic, analytic, and post-analytic processes, such as training, interlaboratory comparison, preventative maintenance, and result reporting.

quality control.
Quality control (QC) is a statistical control for precision and accuracy of laboratory results. Daily QC provides a benchmark to measure the quality of the testing process. When QC falls outside an acceptable range, the laboratory results may not be released.

reagents.
Reagents are the chemical or biological substances used in laboratory testing to detect or measure an analyte (see definition). They vary widely in cost, stability, cold/cool chain requirements, availability, and associated hazards.

reference ranges.
In any given population, the reference range describes the range of normal test results. For example, for adult males the normal glucose range (for a particular technique) will range from 85 mg percent to 110 mg percent. This is considered the normal range for that population.

reliability (or precision).
The reliability of a test is measured in precision. Reliability is not directly related to the sensitivity of the technique but rather to its reproducibility. For example, automated instruments provide more reliability than do manual techniques. As shown by the coefficient of variation (CV)—the measurement of precision reported in percentage—automated instruments typically have a CV of less than 8 percent while manual procedures may have a CV of 15 percent or more.

sharps.
Used needles and lancets, which are biohazardous and medical waste, are called sharps and must be discarded in sharps containers.

sensitivity.
Sensitivity is the probability that a test is positive if the person being tested has the disease or condition. That is, high-sensitivity assays detect a high percentage of true positives. Screening tests, such as rapid HIV tests, must be highly sensitive. Screening tests may require confirmation with a highly specific test, such as the Western blot test.

specifications.
Operational parameters from the manufacturer of a reagent, test, or instrument are defined in the specifications. Specifications may be found in package inserts and instrument manuals. National and international approval of reagents, tests, or instruments is based on meeting those specifications.

specificity.
Specificity is the probability that a test is negative if the person being tested does not have the disease or condition. That is, high-specificity assays detect a high percentage of true negatives. A highly specific test should be used when there is a need to minimize the number of false negatives; for example, when diagnosing an infection in an individual.

standard operating procedures.
Standard operating procedures (SOPs) explain step-by-step how to do a particular test; including specimen requirements, environmental conditions, reference ranges, and reporting units. SOPs should be defined or standardized across each level of the laboratory system. For example, every district lab should have the same set of SOPs for the test techniques carried out at the district level.

standards.
Standards are the concepts, procedures, and designs needed to achieve and maintain the required levels of compatibility, interchangeability, or commonality in the operational, procedural, material, technical, and administrative fields.

standardization.
tandardization (in the laboratory context) is the process of ensuring that the—

* same menu of laboratory tests, defined by level of the laboratory system (central, regional, district), is offered
* same techniques, defined by level, are used to carry out those tests
* same technical SOPs are followed for those techniques
* laboratory instrumentation, defined by level, is agreed upon.

T cell (T lymphocyte).
T cells are white blood cells that stimulate the immune system to fight disease, and they are the primary target of HIV. Called T cells because they mature in the thymus gland, they include T4 and T8 cells, also known as CD4 and CD8 cells.

T cell count (CD4 count).
The T cell count is the number of T4 cells per cubic millimeter (mm 3) of blood—an mm 3 is the size of a pinhead. As HIV disease progresses, the T4 cells fall from a normal count of 500–1,500 to as low as zero. When the T cell (CD4) count goes below 200, the risk of opportunistic infections increases; when the T cell count drops below 50, the risk rises dramatically.

test menus.
Test menus describe the defined list of tests that should be offered at a specific laboratory or level (central, regional, district, etc.) of the laboratory system.

usage.
Usage refers to the amount of laboratory commodities consumed during a set period of time. JSI/DELIVER uses the terms consumption or dispensed to user to describe amounts of health commodities; for example, drugs. In the laboratory, usage is more appropriate because the supplies are not being consumed by or dispensed to a patient but are being used to conduct a laboratory test.

viral load.
Viral load is the measurement of the number of viral particles in the circulating blood. HIV and hepatitis C are often quantified with the viral load test. Viral load and CD4 counts are both predictors of the risk of HIV disease progression. Viral load testing is also used to determine when to initiate or change antiretroviral therapy.

Western blot test.
The Western blot test (WB) is a confirmatory test for the presence of HIV antibodies; it is only performed if the ELISA is positive. The WB can be positive, negative, or indeterminate , which is neither positive nor negative. An indeterminate result usually means that a person has just begun to seroconvert at the time of his or her test. In the rare cases in which this occurs, the person will need to be retested, usually about one month later. False positive results are extremely rare with the WB; the WB confirms that HIV antibodies are present.
Systems in the Laboratory:

hematology.
Hematological information assesses the body’s ability to carry oxygen, provide immunological surveillance, and prevent hemorrhage. Typical tests include complete blood counts, which measure the number of red blood cells, white blood cells, and platelets. Originally, these tests were done by diluting blood and counting cells, and measuring hemoglobin by comparing the color of the blood. Typically, in resource-poor settings, semi-automated instruments are used, with manual backup in the event of stockouts or instrument failure. The instruments used are almost always closed systems, using the manufacturers’ reagents. When generic reagents have been used in the past, quality has suffered and most manufacturers would not support the instruments.

chemistry.
Chemical information assesses the body’s chemical balance. Liver function, kidney function, glucose levels, and enzyme levels are typical chemistry tests. Usually, a specific chemical reaction produces a colored product proportional to its concentration. The instrumentation can range from a very simple filter photometer to an automated testing system. Chemistry tests provide an excellent opportunity to use open systems. Reagent test kits provide high-quality reagents and standards but may be expensive. Consolidated purchasing could provide high-quality reagent kits with competitively priced bulk reagents. Many of these reagents require cold chain.

microbiology.
Microbiological procedures can be either observation of stained specimens by microscope, immunological detection of antibodies to a microbe, or growth and isolation of a microorganism on agar-based media. Typical tests include malaria preps, Widal tests for typhoid antibodies, and stool cultures. These are open systems and can be labor-intensive and relatively low cost. Many microbiological media are very sensitive to absorption of water from the air and require good laboratory practice for storage and reconstitution. In a resource-limited environment, automation is usually not an option.

immunology.
Classic immunological procedures could also be classified as hematological or microbiological tests. However, many new tests are based on detection of antibodies or antigens. Thus, immunology is a growing field with many new tests introduced each year. Tests for classifying white blood cells into CD type can be included as well as viral load procedures.

Classic serological tests, such as the Weil-Felix and Widal tests, are labor-intensive and have some cold chain reagents. The enumeration of CD4 cells and measurement of viral load require sophisticated instrumentation, expensive labile reagents, and a high degree of training. Emerging low-cost, low-tech systems are being introduced into this dynamic field. This area requires very careful analysis of the appropriateness of any technology proposed.

urinalysis.
The testing of urine is a low-technology, labor-intensive part of the laboratory. Test strip technology is used in many resource-poor settings. The test strips require good laboratory practice to prevent premature expiration.