Considerations in Interpreting Diagnostic Test Results

Diagnostic tests are common across the spectrum of cancer care, but results may not always be easily interpreted or applied clinically. In certain types of cancer, related conditions can interfere with the accuracy of diagnostic testing. Thus, it is important to choose the best test, taking into consideration timelines, invasive versus noninvasive testing methods, specificity (a test’s accuracy in identifying negative results), and sensitivity (a test’s accuracy in identifying positive results), two speakers said during a presentation at the ONS 37th Annual Congress.

“Diagnostic tests have specific indications, contraindications, and circumstances that make their results inaccurate or uninterpretable. Use of diagnostic tests should appreciably offer information that can drive clinical decisions,” said Brenda K. Shelton, MS, RN, clinical nurse specialist at the Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins in Baltimore, Maryland. She spoke on the topic with Anna Antonowich- Jonnson, MSN, City of Hope, Duarte, California.

The wealth of diagnostic tests related to cancer care includes lab tests, imaging tests (x-ray, ultrasound, CT, MRI, nuclear scans), and scopes that allow direct visualization of an anatomic area. Each test has a different purpose, and nurses should be aware of ideal indications, as well as contraindications. Among the variables that can confound diagnostic test results are age, gender, fluid balance, body fat, albumin, diet, medications, sexually transmitted diseases, and recreational drugs, Shelton said.

Nurses who perform or prepare patients for diagnostic tests should be familiar with optimal technique, because suboptimal technique can lead to incorrect, incomplete, or questionable/uninterpretable results. Safety issues are also important, as is the need for special handling, she said.

Some specific diagnostic tests that may be needed prior to initiation of cancer therapy are complete blood count (CBC) with differential, chemistry panel, renal function/urine studies (ie, glomerular filtration rate [GFR], serum creatinine test), and hepatic function tests, Shelton said. Equations and formulas for estimating GFR and serum creatinine level are available on the Internet.

To illustrate a scenario in which a series of diagnostic tests would be needed, Antonowich- Jonnson reviewed the case history of a patient with colorectal cancer receiving capecitabine who was found to have anemia. Further tests, she said, would include CBC with differential, B₁₂ level, iron tests, stool exam for guaiac and C. difficile, and possibly, endoscopy and/or colonoscopy.

She said that causes of anemia could include bleeding secondary to mucositis, hemorrhoids, or chemotherapy, and recommended that prior lab tests, medications, iron and ferritin levels, and other prior treatment be reviewed. Regarding guaiac stool testing, Antonowich-Jonnson said that oral iron supplements do not give false-positive results, but that intake of vitamin C, red meat, and dietary peroxidases (ie, turnips and radishes) can lead to false-negative results. She stated that an adequate stool sample is 10 mg/g.

Although some institutions may use several stool samples to evaluate the presence of C. difficile, only one sample is necessary, given the current accuracy of the stool test for toxin, Antonowich- Jonnson told listeners. Patients with diarrhea and suspected infection should be isolated until the diagnosis is established, she said, and outpatients and their families should be educated about appropriate hand hygiene and eliminating risks.

“

Diagnostic tests have specific indications, contraindications, and circumstances that make their results inaccurate or uninterpretable. Use of diagnostic tests should appreciably offer information that can drive clinical decisions.”

—Brenda K. Shelton, MS, RN

According to Antonowich-Jonnson, electrolytes should be assessed and adequate hydration ensured. After that, she said, the patient should be re-evaluated; blood transfusions are generally needed if hemoglobin level is <8, the hematocrit is <23, or the patient is symptomatic.

Shelton and Antonowich-Jonnson reviewed other cases and discussed the considerations involved in selecting several diagnostic tests, including those for dysrhythmias, glucose abnormalities, blood cultures, neutropenia, brain natriuretic peptide, cardiac function, liver function, pulmonary function, EGFR status, and ALK gene rearrangements (for lung cancer).

For any diagnostic test, Shelton said, interpreting the findings should include consideration of status of disease, severity of organ dysfunction, impact of disease status on current choice of treatment, undiagnosed comorbidity, differential diagnosis, additional tests that could be helpful, and essential preparation for diagnostic tests.

The speakers recommended the website www.globalrph.com as a resource to help nurses learn about diagnostic tests and access calculators for body surface area, corticosteroid conversion, opiate equianalgesics, carboplatin area under the curve, creatinine clearance, anion gap, and absolute neutrophil count status.

“In addition to choosing the right diagnostic test, critical thinking and excellent assessment skills are needed,” Shelton said.