Does ALS Show Up in Blood Work?

Standard blood tests cannot diagnose ALS, but a growing set of specialized blood-based biomarkers can flag the disease, help separate it from conditions that look similar, and even predict how quickly it will progress. The most advanced of these is neurofilament light chain, a protein fragment released when nerve cells are damaged, which is now commercially available as a lab test. Routine blood work still plays a supporting role, too, though its value is more about ruling things out and tracking the body’s response to muscle loss than confirming the disease itself.

What Routine Blood Work Can and Cannot Tell You

If you walk into a neurologist’s office worried about ALS, one of the first things they will order is a standard panel of blood tests. These are not designed to diagnose ALS. They are designed to rule out other explanations for your symptoms, such as thyroid disease, vitamin deficiencies, infections, or autoimmune conditions. A normal complete blood count and metabolic panel does not mean you have ALS, and an abnormal one does not mean you don’t.

That said, certain routine markers do behave differently in people with ALS. Creatine kinase (CK), which rises when muscle tissue breaks down, is often elevated. Liver enzymes like ALT and AST tend to run higher too, not because the liver is damaged but because these enzymes are also released from deteriorating muscle. In a large study evaluating routine blood values in ALS patients, CK, ALT, and AST all correlated with the degree of active and chronic denervation visible on nerve conduction studies.¹ These markers rise because muscle is wasting, not because of anything specific to ALS, so they don’t help much with diagnosis. They can, however, offer indirect clues about disease severity.

Creatinine is another routine value worth watching. It reflects overall muscle mass, so as ALS strips away muscle, creatinine drops. Research has shown that low baseline creatinine levels and a rapid decline in creatinine over time both predict shorter survival.² Patients whose creatinine fell fastest had a median survival roughly half that of those whose levels held steadier. A Japanese cross-sectional study also found that faster functional decline tracked with lower creatinine, as well as lower cholesterol, lower uric acid, and higher ferritin levels.³ A multicenter analysis similarly found that higher CK, albumin, and creatinine at diagnosis were associated with better functional status, and higher creatinine was independently linked to lower risk of death.⁴

None of these routine values are diagnostic on their own. A healthy person with naturally low creatinine, or someone with elevated CK from a hard workout, would not be flagged. But when a neurologist is already considering ALS, these values help paint a picture of how much muscle has already been lost and how aggressively the disease is moving.

Neurofilament Light Chain, the Leading Blood Biomarker

The biggest advance in blood-based ALS detection is neurofilament light chain, commonly abbreviated NfL. Neurofilaments are structural proteins that act like scaffolding inside nerve fibers. When motor neurons are injured or dying, NfL spills out into the cerebrospinal fluid and then into the bloodstream, where it can be measured with a simple blood draw.

In ALS patients, blood NfL levels are dramatically higher than in healthy people. One study found that the average serum NfL in ALS patients was about 81 pg/mL, compared with roughly 9 pg/mL in healthy controls.⁵ A multicentre study using plasma found even higher median values in ALS patients, around 217 pg/mL versus 50 pg/mL in healthy controls.⁶ The gap is consistent across studies, though the absolute numbers vary depending on the assay platform used and whether serum or plasma was tested.

Blood NfL testing is now commercially available. A paper reviewing the biomarker qualification process noted that NfL is orderable as a standard laboratory test, though it cautioned that a nuanced understanding of its strengths and limitations is needed to avoid misuse.⁷ Real-world data from a commercial platform confirms that the prognostic value seen in research studies holds up in routine clinical settings.⁸

One practical advantage of NfL is that the protein is remarkably stable. Serum and plasma NfL concentrations hold up for at least seven days at room temperature before processing, with barely measurable drift.⁹ Samples can also be frozen, thawed, and refrozen up to three times without significantly changing the result.¹⁰ That makes it practical for clinics that need to ship blood to a reference lab rather than running the assay on site.

How Well NfL Distinguishes ALS from Look-Alike Conditions

A high NfL level alone does not prove someone has ALS. NfL rises in any condition that damages neurons, including multiple sclerosis, stroke, and traumatic brain injury. The diagnostic question is not “is NfL elevated?” but “is it elevated enough, and in the right clinical context, to point toward ALS rather than something else?”

When researchers tested serum NfL against a prespecified cutoff of about 111 pg/mL in patients referred for possible motor neuron disease, the test correctly identified roughly three-quarters of those who turned out to have ALS (sensitivity of 0.77) while correctly ruling out about three-quarters of those who did not (specificity of 0.75). An optimized cutoff around 82 pg/mL pushed sensitivity to 0.86 while keeping specificity at 0.75. The positive predictive value was high, above 0.90, meaning that a result above the cutoff in this clinical population was usually right. But the negative predictive value was low, around 0.48 to 0.60, meaning a normal NfL could not reliably rule ALS out.¹¹ That study’s title bluntly called NfL’s standalone diagnostic value “limited,” and the numbers bear it out: about one in five to one in four ALS patients had NfL below the cutoff.

Against ALS mimics specifically, the picture improves somewhat. One study comparing ALS patients with people who had neuropathies, myelopathies, and myopathies found that plasma NfL distinguished ALS from these imitators with an area-under-the-curve of 0.83 and a sensitivity of about 76%.¹² A cross-sectional study comparing ALS patients with ALS mimics using both blood and cerebrospinal fluid NfL found similar results, with plasma NfL producing an area-under-the-curve of 0.87.¹³

Serum NfL also clearly separates ALS from a closely related but slower condition called primary lateral sclerosis (PLS). In one referral cohort, the median NfL was about 218 pg/mL for ALS but only 66 pg/mL for PLS.¹⁴ That distinction matters because PLS progresses much more slowly and has a very different outlook.

Predicting Progression and Survival

Beyond diagnosis, NfL levels offer one of the strongest blood-based clues about how fast ALS is likely to progress. In the multicentre biomarker study, plasma NfL at the first clinic visit was the only blood marker independently associated with survival after accounting for clinical variables like age, sex, and where symptoms started. The hazard ratio was about 3.0 for each standard-deviation increase in NfL, meaning substantially higher NfL meant substantially shorter survival.⁶ Higher NfL also correlated with faster functional decline.

Inflammatory ratios from a standard complete blood count may add modest prognostic information on top of NfL. The neutrophil-to-lymphocyte ratio, for example, correlated with faster disease progression in one study, independent of sex, age at onset, and other clinical factors.¹⁵ Plasma levels of certain inflammatory cytokines, including IL-6, also tracked with worse function and faster progression.¹⁶ These markers are nonspecific, though, and no one would use them alone to guide ALS prognosis.

Other Blood Biomarkers Under Investigation

NfL gets the most attention, but it is far from the only molecule researchers are tracking in ALS blood samples. Several others are in various stages of validation.

Phosphorylated neurofilament heavy chain (pNfH) is a close relative of NfL. In ALS patients, median plasma pNfH was about 101 pg/mL versus 16 pg/mL in controls. At an optimized cutoff, pNfH had a sensitivity and specificity of about 75% for separating ALS from controls.¹⁷ An intriguing finding is that pNfH may rise before a clinical diagnosis is made. In one study using pre-diagnostic blood samples, the majority of people who later developed ALS already had elevated serum pNfH up to 18 months before diagnosis, and those higher levels predicted shorter survival.¹⁸ That raises the tantalizing possibility of catching the disease earlier, though the clinical pathway for acting on a pre-symptomatic blood result does not yet exist.

GFAP (glial fibrillary acidic protein) is released by a type of brain support cell called an astrocyte. Serum GFAP is elevated in ALS compared with controls, but its ability to distinguish ALS from healthy people is weaker than NfL’s, with one study reporting an area-under-the-curve of only about 0.62.¹⁹ Where GFAP may shine is in advanced disease. A prospective multicenter study found that elevated GFAP was independently associated with poorer survival in patients already at later stages of ALS, with a hazard ratio close to 7 for those with the highest levels.²⁰ So GFAP looks less useful for early diagnosis and more useful for gauging how aggressive the disease has become.

TDP-43 is the misfolded protein at the heart of most ALS cases. Detecting it in blood has been difficult, but one study reported that plasma TDP-43 levels separated ALS patients from healthy controls with about 91% sensitivity and 91% specificity.²¹ A newer approach isolates tiny packages called extracellular vesicles shed by astrocytes and measures the phosphorylated form of TDP-43 inside them, reporting sensitivity and specificity around 87–89%.²² These results are exciting because TDP-43 is directly involved in the disease process rather than being a general marker of nerve damage, but the assays remain in the research phase and have not been tested against the full range of conditions that mimic ALS.²³

Multi-Marker Panels and Gene Expression Signatures

No single blood molecule perfectly diagnoses ALS, which is why researchers are exploring combinations. A recent proteomics study identified a panel of 17 plasma proteins that, along with age and sex, distinguished ALS from both healthy people and those with other neurological diseases with an area-under-the-curve of about 96%.²⁴ Another group took a different approach entirely, using gene expression patterns from whole blood rather than protein levels. Their machine-learning classifier achieved an area-under-the-curve above 0.96 in internal testing and about 0.89 in an independent external cohort.²⁵

These multi-marker approaches are still in development. Their accuracy in controlled research settings looks promising, but translating a panel of 17 proteins or dozens of gene transcripts into a standardized clinical test requires years of validation across diverse populations. The technology may eventually complement NfL rather than replace it, adding diagnostic confidence in ambiguous cases.

MicroRNAs as Blood Biomarkers

MicroRNAs are tiny regulatory molecules that circulate in blood and can be measured in serum. Several have been found at altered levels in ALS patients. A longitudinal study tracked circulating microRNAs in ALS patients over time and found that high levels of miR-206 and miR-151a-5p early in the disease predicted a slower decline in function, while miR-133a levels were informative at moderate and severe stages.²⁶ An independent study in Egyptian patients confirmed that miR-206 was consistently elevated in ALS blood.²⁷

MicroRNA testing is not yet part of routine ALS care. The assays are more complex than a standard protein measurement, and results can vary depending on the platform used. Their long-term value may be less about making an initial diagnosis and more about staging the disease and predicting its trajectory, since different microRNAs appear to rise or fall at different phases.

Genetic Testing from a Blood Draw

Blood is also the standard source material for genetic testing in ALS. About 5–10% of ALS cases are classified as familial, meaning there is a known family history, and a smaller but real fraction of apparently sporadic cases carry identifiable gene mutations. The most commonly tested genes are C9orf72, SOD1, TARDBP, and FUS.²⁸ Testing options range from targeted assays for the C9orf72 repeat expansion to multi-gene sequencing panels.²⁹

Genetic results matter clinically now in a way they did not a few years ago. The C9orf72 repeat expansion, the most common genetic cause of ALS, can be reliably detected in DNA from blood because there are no documented cases where a blood test missed an expansion that was present in the nervous system.³⁰ And knowing whether someone carries a SOD1 mutation is now directly relevant to treatment, because a gene-targeted therapy exists for that subtype.

Blood Biomarkers in Drug Development and Treatment Monitoring

The approval of tofersen, a therapy aimed at SOD1-ALS, brought blood biomarkers squarely into the treatment landscape. Tofersen was approved based in part on its ability to reduce NfL levels in blood, a first for ALS drug development. That decision reflected a growing consensus that if a drug lowers NfL, it is probably slowing the underlying nerve damage, even before that shows up on clinical scales.³¹

A critical appraisal of NfL’s role in drug development concluded that there is sufficient evidence to support using a reduction in NfL as a pharmacodynamic biomarker to guide go/no-go decisions in phase 2 clinical trials.³² In plainer terms, researchers are now using blood NfL as a quick readout of whether an experimental drug is doing anything meaningful to the disease. That does not mean every drug that lowers NfL will work, but it gives trials a faster signal than waiting months for clinical decline to diverge between treated and untreated groups.

For individual patients, this means that if you are on a gene-targeted therapy or enrolled in a clinical trial, your doctor may track your NfL over time as one indicator of whether the treatment is having an effect. A sustained drop would be encouraging. A flat or rising level would raise concern. The test is not a crystal ball, but it provides a biological measurement where previously doctors had to rely entirely on periodic clinical exams.

Why Blood Tests Have Not Replaced the Clinical Diagnosis

Given all this progress, it is fair to wonder why ALS is still diagnosed primarily through clinical examination, electromyography, and a process of excluding other conditions. The reason is partly statistical and partly biological. NfL, the best-validated blood marker, has high sensitivity when comparing ALS patients against healthy controls, reaching about 89–90% in some studies.³³ But the real clinical question is rarely “ALS or healthy?” It is “ALS or something else that also damages nerves?” Against that harder comparison, performance drops. And because NfL is not specific to ALS, an elevated level in someone with early, ambiguous symptoms could reflect any number of neurological conditions.

The blood-brain barrier adds another layer of complexity. NfL must leak from dying neurons into cerebrospinal fluid and then cross into the bloodstream, a process that varies between individuals and can be influenced by barrier integrity.³⁴ Age also matters: NfL naturally rises as people get older, which narrows the gap between “elevated because of ALS” and “elevated because of aging” in the demographic that gets ALS most often.

The realistic near-term role for blood biomarkers is as an additional tool rather than a replacement. A high NfL result in someone with progressive weakness, fasciculations, and upper motor neuron signs can add confidence to a diagnosis that is already clinically suspected. A normal NfL, on the other hand, should not reassure a patient whose symptoms are otherwise concerning, because the test misses a meaningful minority of cases. Blood markers are best understood as one more layer in a diagnostic workup that still depends on clinical expertise.