Analyzing the chemical composition of cerebrospinal fluid

Your patient's cerebrospinal fluid (CSF) holds secrets that can tell you a lot about their health—if you know what to look for. In this video, from our Body Fluid Lab Essentials course, you'll discover the CSF protein and glucose concentrations that spell trouble for your patient, the simple glucose calculation mistake that can cost you a diagnosis, and the little known test that will tell you whether meningitis is bacterial or viral.

Learn how to obtain bodily fluid samples, order the right tests, and quickly diagnose your patient’s problem with this course.

From time to time, we send various bodily fluids to the laboratory—but are we ordering the right tests on these fluids to come to an accurate diagnosis? After this course, you will be comfortable ordering and interpreting bodily fluid analyses. You will understand the clinical implications of lab abnormalities and will know what to do about them.

Determination of cerebral spinal fluid protein and glucose concentrations are routinely done, and may reveal useful clinical information. The normal CSF protein concentration ranges from 23 to 38 milligrams per deciliter in adults. Elevations in the CSF protein concentration can occur in both infectious and non-infectious conditions, including conditions associated with obstruction of cerebral spinal fluid flow.

CSF protein can be elevated by a subarachnoid hemorrhage, or a traumatic lumbar puncture. The presence of cerebral spinal fluid bleeding results in approximately one milligrams of protein per deciliter, for every 1000 red blood cells present per microliter. Brain tumors, stroke, Guillain-Barre syndrome, and systemic disorders such as hypothyroidism are examples of conditions that cause increased cerebral spinal fluid protein.

The CSF protein concentration may be mildly elevated in patients with diabetes mellitus. Cerebral spinal fluid protein elevations may persist for weeks or months following recovery from meningitis. And while they indicate that something is wrong, they are a nonspecific finding, and have little utility in assessing the response to therapy.

Measuring glucose levels in CSF can also be helpful. Low cerebral spinal fluid glucose concentration may occur in a variety of infectious and non-infectious pathologic conditions. Elevated CSF glucose concentrations only occur in the setting of hyperglycemia. CSF concentrations less than 18 milligrams per deciliter are strongly predictive of bacterial meningitis.

In contrast, the CSF glucose concentration is typically normal during most viral central nervous system infections. Low CSF glucose concentrations can also occur in non-infectious conditions, including patients with leptomeningeal carcinomatosis, leukemia, central nervous system lymphoma, severe subarachnoid hemorrhages, or neurosarcoidosis.

These patients may have cellular or inflammatory infiltrates, that disrupt the active transport of glucose into the cerebral spinal fluid. Hyperglycemic patients who present with central nervous system symptoms may have low CSF glucose concentrations. In the setting of hyperglycemia, low cerebral spinal fluid glucose may not be recognized if only the absolute CSF glucose concentration is considered.

So in the setting of hyperglycemia, the CSF fluid glucose to serum glucose ratio should be calculated. The normal cerebral spinal fluid to serum glucose ratio is greater than 0.6. Determination of the cerebral spinal fluid lactate concentration has been suggested as a useful test to differentiate bacterial from viral meningitis.

The mean CSF lactate level in bacterial meningitis is approximately 16.5 millimoles per liter, where it is only 2.4 millimoles per liter in viral meningitis. Two meta-analysis concluded that the diagnostic accuracy of CSF lactate was superior to that of CSF white blood cell count, glucose and protein concentration in differentiating bacteria from viral meningitis.

Although the sensitivity was lower in patients who received antimicrobial treatment prior to lumbar puncture. Lastly, note that cerebral spinal fluid lactate may be elevated in patients with other central nervous system diseases, such as seizure disorders and cerebral ischemia.