2 Laboratory and Specimen-Based Diagnostics

Interpret results in context

Laboratory findings should be interpreted alongside symptoms, medical history, medications, and prior results—not in isolation. A result outside the reported range may call for consideration or follow-up, but one abnormal result alone may not establish a diagnosis. A result within range does not always rule out disease.

Reference ranges differ by laboratory and may vary with age, sex, pregnancy, and testing method. Compare a result with the range and units printed on that patient’s report rather than relying on a range from another laboratory or a general online source.

Prepare for laboratory tests

Preparation depends on the test, so follow the specific instructions provided. When fasting is ordered, plain water is generally allowed. Patients should disclose medicines and supplements and ask the ordering clinician about medicines; they should not stop medicines unless instructed.

A flagged result slightly above a range does not by itself mean the patient has the disease associated with that test. Interpret it using the report’s own range, the patient’s clinical findings, and other results; follow-up may be needed.

Collect a clean-catch urine specimen

A clean-catch urine specimen helps limit contamination by using the collection technique provided for the test. Clean the area as instructed, begin urinating into the toilet, and then collect urine midstream. Do not touch the inside of the cup or lid, and do not add water if there is not enough urine.

Complete a timed urine collection

For a 24-hour urine collection that begins at a specified time, urinate into the toilet at the start time; the first urine is usually discarded. Collect every urine void after that for the next 24 hours, including the final urine at the same time the next day. Do not discard urine passed overnight.

Follow the laboratory’s instructions for storing and returning the collection. If a sample is missed or spilled, contact the laboratory for instructions because the collection may need to be restarted.

Interpret CBC and CMP findings

A CBC includes measures such as white blood cell count, hemoglobin, hematocrit, and platelet count. Hemoglobin and hematocrit below the laboratory’s reference ranges may be consistent with anemia, but the finding must be interpreted with the patient’s health, other CBC values, and the laboratory’s range. It does not by itself prove a bacterial infection or a clotting disorder.

A CMP includes measures such as creatinine, electrolytes, and glucose. Creatinine is a waste product filtered by the kidneys, so a high result may indicate reduced kidney filtration. It does not confirm kidney impairment on its own, prove dehydration, confirm liver failure, or directly measure platelet function; other factors, tests, and clinical context matter.

Interpret urinalysis findings

Urinalysis findings should be interpreted with symptoms and other relevant results. Nitrites may occur when certain bacteria are present, so a positive result can support suspicion of a urinary tract infection. Some infections do not produce a positive nitrite test, so a negative result does not rule out infection. Symptoms and, when indicated, urine culture help guide interpretation; a urine culture may still be useful when nitrites are present.

Examples of adult reference ranges

These adult values are examples, not universal cutoffs. Laboratories may use different reference intervals and units, so compare each result with the range on the patient’s report.

• White blood cell count (WBC): $4{,}500\text{–}11{,}000\ \text{cells}/\mu\text{L}$

• Hemoglobin: $13\text{–}18\ \text{g/dL}$ (male); $12\text{–}16\ \text{g/dL}$ (female)

• Platelet count: $150{,}000\text{–}400{,}000/\mu\text{L}$

• Sodium: $135\text{–}145\ \text{mEq/L}$

• Potassium: $3.7\text{–}5.2\ \text{mEq/L}$

• Creatinine: $0.6\text{–}1.3\ \text{mg/dL}$

• Glucose: $70\text{–}100\ \text{mg/dL}$ (fasting example)

Respond to symptoms after venipuncture

If a patient feels faint after venipuncture, help them lie down to reduce the risk of falling. Apply firm, direct pressure to the puncture site if it is still bleeding. Reassess the patient and seek assistance if symptoms or bleeding persist.

• Reference range

• Complete blood count (CBC)

• Comprehensive metabolic panel (CMP)