The question of whether a Licensed Practical Nurse (LPN) can read a Tuberculin Skin Test (TST), often called a PPD test, does not have a simple answer. LPNs are licensed caregivers who provide basic patient care under the direction of a Registered Nurse (RN) or physician. The TST is a screening tool used to determine if a person has been infected with Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). The LPN’s ability to perform this task depends heavily on the local laws governed by the State Board of Nursing and the specific policies of the employing healthcare facility.
Understanding the Tuberculin Skin Test Procedure
The TST involves an intradermal injection of purified protein derivative (PPD) into the inner surface of the forearm using the Mantoux method. This injection creates a pale, raised area called a wheal, which is typically 6 to 10 millimeters in diameter if performed correctly. The test is a measure of the body’s immune response, specifically a delayed-type hypersensitivity reaction, to the injected protein.
The results of the test must be assessed between 48 and 72 hours after the injection. If the patient does not return within this window, the results become unreliable, and a new test usually needs to be placed. Reading the test involves inspecting the injection site under good lighting and palpating the area to locate the hardened, raised swelling.
The trained professional must identify and measure only the induration, which is the firm, raised area or thickening of the skin. Erythema, or redness, may also be present but is not relevant to the interpretation of the test result and must be ignored. The measurement is taken in millimeters across the widest part of the induration, perpendicular to the long axis of the forearm. Documenting the precise measurement in millimeters, even if zero, is a requirement for accurate record-keeping.
The Critical Difference Between Measurement and Clinical Judgment
The act of “reading” the TST is often divided into two distinct components: the mechanical measurement and the subsequent clinical interpretation. The physical measurement, which involves palpating the induration and recording its diameter in millimeters, is a technical skill that LPNs are often trained to perform and document. This measurement provides the objective data point required to determine the significance of the reaction. The measurement alone does not determine if the result is positive or negative.
The second component, clinical judgment, requires interpreting that measurement based on the patient’s specific risk factors for tuberculosis. A single measurement, such as 8 millimeters of induration, may be negative for a healthy individual with no known risk factors, but it could be considered a positive result for an immunocompromised patient. The Centers for Disease Control and Prevention (CDC) guidelines establish different positive thresholds—typically 5 mm, 10 mm, or 15 mm—depending on the patient’s individual risk profile.
CDC Positive Thresholds
An induration of 5 millimeters or greater is considered positive in highly susceptible populations, such as people with HIV, recent contacts of active TB cases, and organ transplant recipients. A 10-millimeter induration threshold is typically used for healthcare workers, recent immigrants from high-prevalence countries, and children under four years old. Only an induration of 15 millimeters or more is considered positive for individuals with no known TB risk factors.
Applying these variable thresholds requires a thorough understanding of the patient’s medical history and risk factors, which moves the task from a technical measurement to a complex act of clinical interpretation. Many State Boards of Nursing specify that while LPNs may perform the measurement and documentation, they may not make the final determination of whether the test is positive or negative. This determination is an act of clinical judgment often reserved for the Registered Nurse or an advanced provider. The final interpretation often leads to a follow-up plan, which is a complex decision-making process typically outside the LPN’s scope of practice.
State Boards of Nursing and Delegation Protocols
The definitive answer to whether an LPN can read a TB test is found within the Nurse Practice Act of the state where the LPN is licensed. State Boards of Nursing (BONs) regulate the scope of practice, and they frequently permit LPNs who have demonstrated competency to assess the results, which includes the measurement of induration. This permission is usually granted under the direction and supervision of an authorized health care practitioner or a Registered Nurse.
The concepts of delegation and supervision are central to this practice. An RN or physician may delegate the mechanical function of measuring the induration to the LPN, especially when the LPN has received specific training and validation of competency. Facility policies play an important role and are often more restrictive than state law. A facility might require the LPN to measure and document the induration size in millimeters but mandate that an RN or physician conduct the final interpretation of the result and sign off on the follow-up plan.
The LPN’s role is primarily data collection and documentation, with the responsibility to communicate the measured result to the supervising licensed professional for the final clinical determination. This structure ensures that the complex interpretation, which requires balancing the measured induration size against the patient’s risk profile, is performed by the professional whose scope of practice explicitly includes that level of clinical judgment. Written facility policies must also outline clear referral processes for positive findings or for induration measurements that fall within the positive range for high-risk populations.