1 Foundations of Laboratory Interpretation

Interpret results in context

Laboratory results are clues, not diagnoses by themselves. Interpret each result in context by checking the test, units, collection time, specimen type, the reporting laboratory’s reference interval, the patient’s condition, and previous results.

A result marked high or low should prompt assessment rather than an automatic conclusion. Consider how far it is from the applicable interval, whether it fits the patient’s symptoms and other results, and whether the finding is new or persistent.

Reference intervals and decision limits

A reference interval describes results expected in a defined reference population. Intervals may differ by laboratory, method, age, sex, pregnancy, and other characteristics. Many represent the central $95\%$ of a healthy reference group; therefore, a healthy person may fall outside the interval, and a person with illness may have a result within it. Use the interval printed on the report rather than one from another laboratory.

A clinical decision limit is different: it is a threshold used to guide a clinical decision, not simply a statistical reference interval. An out-of-range result alone does not establish a diagnosis.

For example, a sodium result of $133\ \text{mmol/L}$ is below the reporting laboratory’s interval of $136\text{–}144\ \text{mmol/L}$. The appropriate interpretation is that the result is below that laboratory’s interval and needs to be considered alongside the patient’s condition and other information—not that it establishes a specific diagnosis or that another laboratory’s interval should replace the one on the report. Confirm the units and context.

Compare results over time

Compare results with earlier results using the same units and, when possible, the same laboratory. A result may remain inside the reference interval while changing substantially from the patient’s baseline. Conversely, a small isolated out-of-range result may be less meaningful than a repeated or worsening change.

When comparing results, review collection times, recent treatment, medication changes, and other relevant circumstances. If a patient’s creatinine is higher than on prior tests but remains within the printed interval, recognize the change from baseline, compare dates and units, review clinical context and recent treatments, and communicate a meaningful or concerning trend through the appropriate clinical channel. Being “within range” does not make a change irrelevant.

Specimens and factors that affect results

A result is only as reliable as its specimen. Follow the test-specific instructions for patient preparation, tube or container, fill volume, mixing, transport, storage, and processing. Confirm patient identification and specimen labeling. If the specimen is unsuitable or the result conflicts with the clinical picture, follow laboratory and facility procedures for clarification or recollection.

Factors that can affect results include:

• Patient factors: age, pregnancy, recent food or drink, fasting status, medications and supplements, strenuous exercise, stress, and time of collection. Follow the ordered test’s preparation instructions; do not assume every test requires fasting.

• Collection factors: prolonged tourniquet use, fist pumping, a difficult or traumatic draw, collection near an IV infusion, and use of the wrong tube. These can alter a result or contaminate the sample.

• Specimen condition and handling: hemolysis, clotting when an anticoagulated sample is required, delayed separation from cells, incorrect temperature, and transport delay can affect some tests. Hemolysis, for example, can falsely raise measured potassium.

Respond to unexpected results

If a result is unexpected, assess the patient and the urgency of the finding first. Then review specimen comments and collection conditions, and contact the laboratory or responsible clinician according to policy. Do not assume that a possibly compromised result is harmless, and do not let a suspected collection problem delay escalation of a concerning result or patient condition.

For an unexpectedly high potassium result from a hemolyzed specimen, assess the patient and urgency, notify the responsible clinician according to policy, and review whether the specimen may have affected the result. Hemolysis can falsely elevate potassium; recollection or confirmation should follow the clinician’s direction and local protocol. Do not independently dismiss a concerning result.

If a patient exercised strenuously shortly before a blood draw and did not follow ordered fasting instructions, consider that exercise and preparation can affect some tests. Document the circumstances, check the test’s preparation requirements, and ask the responsible clinician or laboratory whether interpretation, repeat testing, or another action is appropriate. Do not assume every test requires fasting.