Lab tests: Just what the doctor ordered

Issue: March 1996

ANAHEIM, Calif.—For primary care optometrists, laboratory tests can be useful assets in helping to develop differential diagnoses and justify therapeutic regimens.

Harue J. Marsden, OD, in a presentation here during the International Vision Expo, said lab testing can be a big factor in helping an optometrist develop a treatment rationale: "The medicolegal aspects become important when you are trying to justify why you have implemented a certain therapeutic regimen."

While many tests should not be relied upon individually, Marsden said, "You use them with many of your clinical findings to support data to come up with your diagnosis." She noted that is important to specify the age and sex of the patient.

Panels simplify ordering

When ordering tests, Marsden said the process is usually simplified with a straightforward form. Also, tests are often grouped together in panels, she said. "A panel of tests is nice because these tests are the most common that would be ordered to investigate or evaluate a particular disease entity."

Marsden said interpreting the data is made easier because the lab will send a data sheet with normal ranges. The lab will usually "throw up a flag and say, 'This value is outside the norm,'" she said.

Optometrists should determine how much responsibility they will take when entering into a relationship with a lab. "Specimen collection is important because much patient preparation is needed in certain tests," she said.

For example, Marsden said, you must remember to tell patients being tested for fasting blood sugar that they cannot eat prior to the test. If they don't receive the instructions, money and lab time will be wasted, she said, "So you want to confirm whether or not you want to take that responsibility."

For patients, the tests can simplify the transition between care givers, she said. If the OD suspects disease, patients can be sent for testing so they "will have the tests accompanying them to the internal medicine physician or family practice physician, whoever is seeing them for that particular entity."

Marsden suggested that optometrists seek lab tests if they suspect or encounter the following:

Recurrent uveitis. Testing can be considered for recurrent or severe uveitis, or for uveitis associated with systemic signs. The primary tests are the complete blood count, antinuclear antibody, rheumatoid factor, erythrocyte sedimentation rate (ESR), fluorescent treponemal antibody absorption, angiotensin-converting enzyme, purified protein derivative, human leukocyte antibody and enzyme-linked immunosorbent assay for Toxocara.

There are many uveitis tests, Marsden said, because they are nonspecific: "You usually have to order multiple tests to confirm or rule out a particular systemic entity."

She cited research that indicated almost 50% of all uveitis "has some systemic underlying disease to cause it. So lab tests can be very useful to determine what that systemic etiology might be."
Diabetes mellitus. If patients have a fluctuating refractive error (not always to the myopic side) and weight loss, and they report increased hunger or urination, the following tests can help screen for diabetes: fasting blood sugar (the most commonly ordered test for this) and 2-hour postprandial glucose. The glycosylated hemoglobin test can help if the OD suspects poor compliance in a long-standing diabetic.
Temporal arteritis. Symptoms are often nondistinct, but include headache, fever, a tender temporal artery combined with transient or sudden vision loss, diplopia and optic atrophy. The recommend test is the ESR.

Marsden said the test is nonspecific, but "if there is an equivocal finding or something in that range, you can play it safe and start steroid therapy."
Hyperthyroidism. Signs and symptoms include lid retraction, exophthalmos, orbital congestion and optic neuritis. This panel of tests includes total T4, T3 uptake, T3 radioimmunoassay, T3 radioimmunoassay and free T3 or T4 index.
Elevated blood lipids. Patients are suspect if they display corneal arcus, particularly if they are younger than 40; xanthelasma; and lipemia retinalis. Panels of cholesterol and triglycerides tests can be ordered.
AIDS. The signs and symptoms are standard: recurrent fever, swollen lymph glands, constant fatigue, drastic weight loss, AIDS retinopathy, cytomegalovirus retinitis and toxoplasmosis retinitis. Marsden suggested also to look for tuberculosis. The two main tests are the enzyme-linked immunosorbent assay and the Western blot.

"The CDC [Centers for Disease Control and Prevention] says you cannot just order one of these and say the patient does or does not have AIDS," said Marsden. She said if AIDS is suspected, "The individual must be sent to a primary care physician with your clinical impression."