What is SII (inflammation index)?
Neutrophils and lymphocytes respond in opposite ways to the same signal: adrenaline and cortisol put neutrophils into the blood and send lymphocytes back to the lymph nodes, and with an infection or an inflammation the same happens. Platelets join in, but more slowly. An inflammation that lasts for days sets the bone marrow to making more platelets, and those stay in the blood for one to two weeks. That is how the SII bundles a fast signal and a slow signal into one number. That is the idea behind the index. Platelets times neutrophils sits above the line, lymphocytes below it, and so the SII contains both the NLR and the platelet/lymphocyte ratio: everything that lifts the neutrophils or the platelets, or suppresses the lymphocytes, lifts the index. In a healthy adult at rest it lands in the low hundreds. With a severe bacterial infection it runs into the thousands, and in someone who sleeps badly, trains a lot or smokes, it creeps up slowly. Your report does not print the index; you calculate it yourself with the calculator on this page, from the three absolute counts in ×10⁹/L. The one study that has published a reference interval, in 5,969 healthy adults aged 18 to 79, comes out at 142 to 804, and above 65 the index sits somewhat lower (Luo et al., 2019).
Why is SII (inflammation index) relevant?
The SII shows how heavily your body is loaded, like the NLR, with one difference: the platelets. Neutrophils live a day and are quickly back at their usual level after a short load; platelets rise over days with an inflammation and fall over weeks. A load that has lasted a while, a cold that lingers or a smouldering inflammation, therefore keeps the index high while the NLR is already normal again. The two together say whether something is acute or has been running for a while. The second reason is the link with cardiovascular disease in the general population. In a group of 42,875 American adults followed for up to 20 years, people with an index above 655.56 had a 29 percent higher chance of dying and a 33 percent higher chance of a cardiovascular death than people below 335.36 (Xia et al., 2023). That is an association and not a cause: the index measures the smouldering inflammation of belly fat, smoking and little exercise, and it falls along when lifestyle changes. That makes it a usable measure to follow over the years, beside CRP, which measures the inflammation itself. The honest limitation is that the index does not say which of the three numbers moved. An SII of 900 can come from a bacterium that lifted the neutrophils, from an iron deficiency that raised the platelets, or from prednisone that suppressed the lymphocytes. That is why you always read it beside the three separate numbers, beside the NLR and beside CRP, and why one result is a snapshot and the trend the real number.
SII (inflammation index) reference ranges
What counts beside the number:
- The platelets, neutrophils and lymphocytes themselves, because the index does not say which of the three moved
- CRP from the same draw, which says whether inflammation is running or load lifted the index
- The NLR from the same draw, because it responds within a day and the platelets only over days
- How you slept, how hard you trained the day before and whether you take prednisone
- Your earlier results, because the trend says more than one number
- Your ancestry and your iron status, because a naturally lower neutrophil count gives a low index, and an iron deficiency lifts the platelets
No lab prints the SII and there is no national reference value; what exists is one study that publishes an interval, 142 to 804, and a population study that links the top quarter, above 655.56, to more deaths over 20 years. That second number is not a limit for your result: a cold or a hard training week puts you over it temporarily. A rested body usually sits in the low hundreds, and the index itself is a calculation you make from the same draw.
SII = platelets × neutrophils ÷ lymphocytes, all three in ×10⁹/L
Multiplying and dividing is all there is: no constant to account for. Two counts above the line and one below leave one ×10⁹/L, so the index carries the unit of a cell count, and the literature mostly writes it as a bare number. All three figures come from the same blood count, so nothing extra is drawn for the index.
× ÷ =
SII (inflammation index) high or low: what it means
A low SII means the lymphocytes dominate relative to the platelets and the neutrophils, and that is usually favourable: the balance lies towards recovery. Below 142 the question is which side it comes from. More lymphocytes is usually a virus or the recovery phase after it. Fewer neutrophils is a virus that has just passed, a medicine, or a naturally lower count, which is common in people with African or Middle Eastern ancestry. Fewer platelets suppress the index too, and then the platelet count itself is the value to look at. A high SII means your body is in the stress state or an inflammation is running, and the question is why. Above 804 with fever and a high CRP it is an infection or an inflammation, and then the index falls with recovery, only more slowly than the NLR, because the platelets lag behind. Without fever it is almost always load: a hard training session the day before, a bad night, a period of pressure, smoking, prednisone or belly fat. An index that stays at the high end over several measurements without an infection is the smouldering inflammation, and you read that beside your waist, your sleep and your CRP. What you do about it: with a rise from an infection nothing but letting it pass, and measure again only two to four weeks later, because the platelets fall slowly. With an index that sits high from load, sleep, rest days and dosing the training are the levers. With an index that structurally sits at the high end without an infection, stopping smoking, less belly fat and regular exercise are what bring it down over months. Have blood drawn on a rest day after a good night, and calculate the index each time from the same draw. The SII belongs beside the platelets, neutrophils and lymphocytes themselves, because the index does not say which of the three moved, beside the NLR, which responds faster and leaves the platelets out, and beside CRP, which says whether inflammation is running.
What lowers it
The lymphocytes dominate, and the question is which of the three numbers moved.
| Cause | How often |
|---|---|
| Fewer neutrophilsA virus that has just passed, a medicine, or a naturally lower count, which is common in people with African or Middle Eastern ancestry | Often |
| More lymphocytesA viral infection or the recovery phase after it; the index is then temporarily low and comes back over weeks | Often |
| A lower platelet count in the numeratorA virus, alcohol, a medicine or make-up; the index falls along, and the platelet count itself is then the value to read | Sometimes |
What raises it
Your body is in the stress state or an inflammation is running, and with or without fever makes the difference.
| Cause | How often |
|---|---|
| More neutrophilsA bacterial infection, an inflammation, a hard training session, smoking or prednisone; adrenaline and cortisol put the cells into the blood | Often |
| Fewer lymphocytesCortisol pulls them out of the blood: with lack of sleep, acute stress, an operation, a severe infection or prednisone | Often |
| A higher platelet count in the numeratorAn inflammation that has been running for days, an iron deficiency or recovery after blood loss; they rise slowly and fall slowly, and keep the index high for weeks | Sometimes |
How does a SII (inflammation index) blood test work?
- Referral
- Not needed, and there is no separate test: the SII is a calculation from the blood count with differential, which you can have measured yourself.
- Fasting
- Not needed; the three numbers do not respond to food. They do respond to exertion, stress and sleep, so come to the draw rested and calm.
- When
- On a rest day after a good night, and at least two weeks after an infection if you want to know your usual level, because the platelets fall slowly. Measure each time under the same conditions.
- The draw
- One tube of blood from a vein in your arm. Platelets, neutrophils and lymphocytes come from the same blood count, and CRP can come from the same draw. Calculate the index with the calculator on this page.
- Repeating
- With a high result after two to four weeks, when you are well and rested. Anyone who wants to follow their load measures two to four times a year under the same conditions.
- Which test
- SII (inflammation index) is in the comprehensive test (€229) and in .
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
What is the SII?
Your platelet count times your neutrophil count, divided by your lymphocyte count, all three from the same blood count. Neutrophils and platelets rise with stress and inflammation, lymphocytes fall, and the index amplifies that difference. The name stands for systemic immune-inflammation index, and the calculation comes from oncology, where it was devised in 2014. It is a calculation, not a separate test.
Is there a normal value for the SII?
No lab band, because no lab measures the index. One study in 5,969 healthy adults publishes an interval of 142 to 804, and above 65 the index sits somewhat lower. A rested body usually sits in the low hundreds. More important than the interval is your own trend: a rise over a few measurements under the same conditions means something has changed.
What does a high SII mean?
That your body is in the stress state or an inflammation is running. With fever and a high CRP it is an infection, and then the index can run into the thousands and falls with recovery. Without fever it is almost always load: a hard training session, a bad night, a period of pressure, smoking, prednisone or belly fat. An index that stays at the high end without an infection goes with smouldering inflammation.
What does a low SII mean?
That the lymphocytes dominate, and that is usually favourable. A temporarily low index fits a virus or the recovery phase after it, in which the lymphocytes rise. A persistently low index is simply make-up in many people with African or Middle Eastern ancestry, because their neutrophil count is naturally lower. If it falls through few platelets, that count itself is the value to read.
How does the SII differ from the NLR?
In the platelets. The NLR divides neutrophils by lymphocytes and responds within a day to stress and infection. The SII multiplies that ratio by the platelets, which rise over days with an inflammation and fall over weeks. That is why the SII stays high while the NLR is already normal again, and together they say whether a load is acute or has lasted a while.
Why is my SII still high after a cold?
Because the platelets lag behind. An inflammation sets the bone marrow to making more platelets, and those stay in the blood for one to two weeks after the neutrophils have already fallen. So measure again only two to four weeks after an infection if you want to know your usual level, and compare measurements you made under the same conditions.
Does the SII say anything about my cardiovascular risk?
At population level it does: in a group of 42,875 American adults, people with an index above 655.56 had a higher chance of a cardiovascular death over 20 years than people below 335.36. For one person it is a measure of smouldering inflammation that moves with belly fat, smoking and exercise, and that you follow over years beside CRP, LDL and non-HDL. It is not a diagnosis.
Can I calculate the index myself?
Yes: platelets times neutrophils, divided by lymphocytes, all three as absolute counts in ×10⁹/L and from the same draw, with the calculator on this page. Never use the percentages of the differential, because those also shift when another cell type moves. Have blood drawn on a rest day after a good night, and calculate the same way each time.
Which tests include SII (inflammation index)?
SII (inflammation index) is in the comprehensive test (€229) and in .
Related biomarkers
Sources
- 1.Hu B, Yang XR, Xu Y et al., Systemic immune-inflammation index predicts prognosis of patients after curative resection for hepatocellular carcinoma, Clinical Cancer Research. 2014. pubmed.ncbi.nlm.nih.gov
- 2.Luo H, He L, Zhang G et al., Normal Reference Intervals of Neutrophil-To-Lymphocyte Ratio, Platelet-To-Lymphocyte Ratio, Lymphocyte-To-Monocyte Ratio and Systemic Immune Inflammation Index, Clinical Laboratory. 2019. pubmed.ncbi.nlm.nih.gov
- 3.Xia Y, Xia C, Wu L et al., Systemic Immune Inflammation Index (SII), System Inflammation Response Index (SIRI) and Risk of All-Cause Mortality and Cardiovascular Mortality, Journal of Clinical Medicine. 2023. pubmed.ncbi.nlm.nih.gov
- 4.LESA Laboratory diagnostics (Dutch primary-care laboratory agreement). 2026. richtlijnen.nhg.org
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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