Hematopoietic subsyndrome: cytokines and support

A 41-year-old operator returns to the outpatient clinic 10 days after spending a few minutes near a cobalt source that fell from industrial equipment. In the first week, he had nausea and vomiting for one day and then felt fine. The dose estimated from the lymphocyte drop and physical dosimetry is about 4 Gy to the whole body. Now he has no complaints, but neutrophils have dropped to 900 per microliter and platelets to 60,000. He asks why he needs to be hospitalized if he feels fine. The scene is fictional and was written for teaching.

This text uses the case as a clinical reasoning scenario for students and health professionals. The topic is the hematopoietic subsyndrome of acute radiation syndrome: what happens to the bone marrow, when and how to use growth factors, what the evidence base is, and how to manage infection, bleeding and transfusion. Initial triage by lymphocytes is in the post on acute radiation syndrome and lymphocyte triage, and is not repeated here. If you play the case of the day in NEXO, you will recognize the challenge: the patient who is well in the latent phase is precisely the one who needs the most attention.

Key points

• The latent phase deceives: the patient feels well while the marrow stops producing.
• REMM recommends myeloid growth factor as early as possible, generally within the first 24 hours after the end of exposure, for estimated doses of 2 Gy or more.
• Filgrastim, pegfilgrastim, sargramostim and romiplostim were approved by the FDA for this indication under the Animal Rule, without human trials.
• Infection prevention, irradiated and leukoreduced blood components, and clinical support are as important as the medication.
• Stem cell transplantation is for few patients, when marrow recovery is unlikely.

Why this is a reasoning scenario

The scene tests a dangerous intuition: if the patient is well, he can go home. In the hematopoietic subsyndrome, the latent phase is the period when the marrow has already been damaged, but the mature blood cells are not yet depleted. A review on the effects of radiation on the human body recalls that hematopoietic syndrome leads to bleeding and infection and increases the risk of death (Talapko et al., 2024). The post on clinical deterioration helps avoid trusting the apparent good condition.

REMM recalls that, as the whole-body or partial-body dose increases, the severity of the hematopoietic subsyndrome also increases (REMM, hematopoietic subsyndrome). In a mass-casualty event, it is unlikely that most people are wearing a dosimeter, and the estimate depends on biodosimetry and on reconstructing the dose from location, time and shielding (REMM, hematopoietic subsyndrome). The post on diagnostic uncertainty helps decide with an estimated rather than measured dose.

What the scene teaches

The scene trains six habits. The first is recognizing the latent phase. The second is starting growth factor early. The third is knowing the approved medications and the origin of their dose. The fourth is understanding the strength and limit of the evidence. The fifth is preventing infection and using the right blood components. The sixth is knowing when to consider transplantation.

What is not at stake

The text does not replace the package insert or the specialist. The doses cited come from REMM and should be checked against the current package insert before any prescription. The text also does not discuss radiation in space or cancer radiotherapy. The post on clinical decision-making helps organize management.

What happens to the marrow

The marrow empties

The review on the effects of radiation explains that hematopoietic syndrome results from damage to bone marrow and lymphoid tissue stem cells, with an overall drop in hematopoietic cells (Talapko et al., 2024). The marrow comes to have more fat cells, which are less sensitive than stem cells, and the next stage of failure is the inability to produce blood cells, especially those that control immunity (Talapko et al., 2024). Organs with rapid division, such as the intestine and marrow, are damaged more easily than muscles and tendons (Talapko et al., 2024).

The review on stem cells notes that the METREPOL system divides hematopoietic subsyndrome into four grades, from H1 to H4, based on peripheral blood cells in the 60 days after irradiation (Qian and Cen, 2020). The same review notes that the Chernobyl accident, on April 26, 1986, led to the hospitalization of 237 people with suspected severe exposure, and that acute syndrome was not confirmed in 103 of them (Qian and Cen, 2020).

Neutrophils and the nadir

REMM has illustrations of the effects of radiation on blood cell counts over time, with curves showing the drop in lymphocytes, neutrophils and platelets (REMM, hematopoietic subsyndrome). A modeling study in primates notes that the timing of deaths relative to the neutrophil nadir was quite different between studies: in the filgrastim study, four of the five animals that died were at the nadir itself, and in the pegfilgrastim study, 11 of the 12 animals that died in the placebo group died after the nadir (Harrold et al., 2020b).

The operator at the scene

With an estimated dose of 4 Gy, neutrophils of 900 and platelets of 60,000 on day 10, the operator is entering the risk phase. The drop will continue, and the nadir has not yet arrived. Hospitalization is not to treat what he feels, but to protect him from what is coming. The post on signs, symptoms and history helps explain this to the patient.

When to start the growth factor

The sooner, the better

REMM recommends myeloid growth factor as early as possible after expected or confirmed exposure, usually within the first 24 hours after the end of exposure (REMM, myeloid cytokines). For filgrastim and pegfilgrastim, REMM advises not to delay the first dose if a blood count is not available right away (REMM, myeloid cytokines). The review on G-CSF notes that the early effects of growth factors after irradiation were interpreted as action against apoptosis, which led to the principle that the sooner, the better (Hofer et al., 2014).

In the practice of accidents, this did not always happen. A 2015 review reports that colony-stimulating factors were used in victims of 16 radiological and nuclear accidents, with benefits observed, but that in only three of them did use begin within the first 48 hours, and in the others it began weeks later (Singh et al., 2015). The post on diagnostic pause helps avoid waiting for the nadir to decide.

And if the first day has already passed

A study in primates with partial-body irradiation, preserving about 5 percent of the marrow, tested filgrastim starting on day 1, day 3 or day 5 (MacVittie et al., 2015). At 11 Gy, all schedules reduced neutropenia compared with the control (MacVittie et al., 2015). The scene of the operator, on day 10, shows the real problem: many patients arrive late, and the decision to start the drug at this stage belongs to the specialist, based on the estimated dose and the count.

When to stop

For filgrastim, REMM advises continuing the daily dose until neutrophils are above 1000 per cubic millimeter on 3 consecutive blood counts, or exceed 10,000 after the nadir caused by radiation (REMM, myeloid cytokines). The blood count should be repeated roughly every 3 days until that point (REMM, myeloid cytokines).

The four approved drugs

The REMM doses

REMM summarizes the regimens from the package insert for the indication of acute exposure to myelosuppressive doses of radiation. The table below repeats these regimens for adults. Pediatric doses and regimens for drug shortages are in REMM and in each product's package insert.

Drug	Adult regimen in REMM	When to start	Note
Filgrastim	10 mcg/kg once a day, subcutaneously	As early as possible, with an estimated dose of 2 Gy or more	Continue until neutrophils are above 1000 on 3 consecutive blood counts
Pegfilgrastim	2 doses of 6 mg subcutaneously, 1 week apart, from 45 kg	As early as possible, with an estimated dose of 2 Gy or more	Specialists recommend not giving the 2nd dose if neutrophils go above 5000
Sargramostim	7 mcg/kg once a day, subcutaneously, above 40 kg	As early as possible, with an estimated dose of 2 Gy or more	Higher doses per kg for smaller children
Romiplostim	10 mcg/kg as a single dose, subcutaneously	As early as possible	The package insert advises giving it even without a blood count

Table source: (REMM, myeloid cytokines). REMM notes that no prospective, randomized clinical trial in humans has proven the efficacy or long-term safety of myeloid growth factors for this indication (REMM, myeloid cytokines). If the drug is in short supply, REMM describes strategies such as starting filgrastim at 5 mcg/kg per day in adults and stopping when neutrophils reach 5000, returning to the standard dose as soon as stock is available (REMM, myeloid cytokines).

The approval dates

A 2023 review of drugs approved for reactor blasts summarizes the sequence: G-CSF was approved by the FDA for the hematopoietic subsyndrome in 2015, GM-CSF in 2018 and romiplostim in 2021 (Bennett et al., 2023). The same review notes that G-CSF was first approved in 1990 for chemotherapy neutropenia, GM-CSF in 1991 and pegylated G-CSF in 2002 (Bennett et al., 2023). According to the FDA lookup made in the post about acute radiation syndrome, the filgrastim indication is dated 03/30/2015, the pegfilgrastim one 11/13/2015, the sargramostim one 03/29/2018 and the romiplostim one 01/28/2021.

The stockpiles

The 2015 review notes that filgrastim, sargramostim and pegfilgrastim had already been used off-label in victims of radiological accidents, and that the United States government bought filgrastim and sargramostim for the strategic national stockpile, regardless of approval (Singh et al., 2015). The 2023 review adds that, between 2013 and 2015, the use of G-CSF for this indication would have depended on emergency use authorization (Bennett et al., 2023).

Where the evidence comes from

The Animal Rule

Because of ethical limits, it is not possible to run clinical trials in humans exposed to lethal doses of radiation, and FDA approval of these drugs can follow the requirements of the Animal Rule (Harrold et al., 2020). The rule requires well-characterized animal models and a significant increase in survival as the primary endpoint (Farese et al., 2013). The post about interprofessional clinical reasoning notes that understanding the origin of the evidence is part of prescribing.

The filgrastim study in primates

In the pivotal study, rhesus monkeys received 7.5 Gy of whole-body irradiation, close to the lethal dose for half the animals at 60 days, and filgrastim started within the first 24 hours (Farese et al., 2013). Overall 60-day mortality fell from 59.1 percent in the control group to 20.8 percent in the treated group (Farese et al., 2013). All animals received clinical support, with fluids, antibiotics, analgesics, antidiarrheals, antipyretics, antiemetics, gastric protectors, nutrition and transfusions (Farese et al., 2013).

The modeling study notes that daily filgrastim at 10 micrograms per kg reduced the duration of grade 4 neutropenia by 4.3 days in one study and by 8.9 days in another, and that weekly pegfilgrastim for 2 weeks reduced that duration by 5 days in one study and by 14.1 days in another (Harrold et al., 2020b).

From monkey to human

A modeling study used data from primates, healthy volunteers and chemotherapy patients with neutropenia to predict the survival benefit of filgrastim in adults and children exposed to radiation (Harrold and colleagues, 2020). The REMM explains that the human dose of 10 mcg/kg was chosen because exposure to the drug at this dose should exceed that of primates treated with the same dose per kg (REMM, myeloid cytokines). For romiplostim, the REMM reports that, in the study in irradiated animals, 60-day survival was 72.5 percent with the drug and 32.5 percent in the control group (REMM, myeloid cytokines).

The limit of the evidence

This is the most important teaching point of the section. The evidence is strong in animals with full clinical support and weak in humans, which comes from accident reports without a control group. The post on differential diagnosis helps separate what is known from what is assumed.

Infection, blood and isolation

Infection

In primates, clinical support alone already increases survival in animals exposed to lethal doses (Farese and colleagues, 2013). In the patient in the scene, this means protective isolation, hygiene, antibiotics if there is fever with neutropenia and prophylaxis as directed by the specialist. The REMM has a page on the management of febrile neutropenia as part of the care of the hematopoietic subsyndrome (REMM, hematopoietic subsyndrome). In the report of the cobalt accident in China, the patient received G-CSF, selective bowel decontamination, prophylactic antibacterial and antifungal agents and reverse isolation (Guo and colleagues, 2014).

Transfusion

The REMM notes that transfusion-associated graft-versus-host disease is a consequence that threatens the life of at-risk patients, including those who received a significant whole-body dose (REMM, blood components). The fresher the blood, the greater the risk, especially in the first 3 days after donation, and most patients die of multiple organ failure in the first month (REMM, blood components). Filter leukoreduction is not enough to prevent this complication, but it is recommended because it reduces febrile reactions, platelet alloimmunization and cytomegalovirus infection; irradiation of the blood prevents graft disease (REMM, blood components).

Whole blood, packed red blood cells, including leukoreduced and frozen units, and platelets should be irradiated (REMM, blood components). The REMM recommends irradiation and leukoreduction when both are available, but notes that, if transfusion is urgent and irradiated blood is not available soon, one should not hesitate to transfuse non-irradiated blood (REMM, blood components).

Platelets and bleeding

The drop in platelets follows that of neutrophils with some delay, and bleeding is the other face of bone marrow failure (Talapko and colleagues, 2024). Romiplostim acts on the thrombopoietin receptor and was approved for this indication in 2021 (Bennett and colleagues, 2023). In the scene, a platelet count of 60,000 on day 10 calls for vigilance for bleeding of the skin, gums and intestine, avoiding intramuscular injections and unnecessary procedures and planning transfusion of irradiated platelets if there is bleeding or a very low count, according to the service protocol. The post on diagnostic uncertainty notes that the count alone does not decide on its own.

Transplant

The REMM recommends considering stem cell transplantation if bone marrow recovery is unlikely, and notes that most irradiated people will not be candidates (REMM, stem cell transplantation). Transplantation requires transfer to a specialized center, and the Radiation Injury Treatment Network should help coordinate transplants in the United States (REMM, stem cell transplantation). If transplantation is considered, a compatible HLA donor must be sought, which helps prevent rejection and graft disease (REMM, stem cell transplantation).

Human data are scarce. In Kyiv, nine Chernobyl victims received allogeneic marrow cells by injection into the bone, with a median of 26 days after exposure, and the median time for granulocytes to rise above 1000 per microliter was 17.5 days in the transplant recipients and 26 days in the 59 controls (Klymenko and colleagues, 2011). In the 2008 accident in China, a patient with an estimated dose of 14.5 Gy had rapid marrow recovery with a haploidentical transplant and mesenchymal cells, but died on day 62 of multiple organ failure (Guo and colleagues, 2014). A 2024 review notes that cell therapies have shown benefit in several animal models, but that it is still unknown whether they will benefit human patients (Christy and colleagues, 2024).

How the scene ends

The operator is admitted to a protective isolation room. Hematology discusses with toxicology and starts the growth factor according to the package insert, with serial blood counts. His HLA typing and his siblings' are collected as a precaution. On day 14, he has fever with neutrophils below 500, receives broad-spectrum antibiotics in under 1 hour and a transfusion of irradiated, leukoreduced platelets. The marrow starts producing again in the fourth week, and transplant is not needed. The post on combined radiation injury and trauma shows how management changes when there is a wound or burn.

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by looking for the ICD-10 code. The hematopoietic subsyndrome is a good exercise because it forces you to act before the patient worsens. The post on learning based on clinical cases explains why this format fixes knowledge.

Three exercises with cases

Change the timing. Say what you would do if the operator arrived 6 hours after exposure, with vomiting and no blood count.

Change the dose. Say how the conversation about transplant would change if the estimated dose were 9 Gy.

Change the setting. Say how you would use the filgrastim stock if there were 200 victims and medication for 50, consulting the REMM strategy.

The post on simulation of care helps set up these exercises. The post on dirty bomb, triage and decontamination shows the first response in an event with many victims.

The limits on using the game are described on the NEXO economy page. If you research teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information on curation and on creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Why admit a patient who is well?

Because in the latent phase the marrow has already been damaged, and the drop in blood cells will still continue (Talapko and colleagues, 2024).

When should the growth factor start?

As early as possible, generally in the first 24 hours after the end of exposure (REMM, myeloid cytokines).

Have these medications been tested in humans?

Not in clinical trials. They were approved under the Animal Rule, based on animal studies (Harrold and colleagues, 2020).

Can I transfuse non-irradiated blood?

If the transfusion is urgent and there is no irradiated blood, one should not hesitate (REMM, blood components).

Who should receive a marrow transplant?

Few patients, when marrow recovery is unlikely (REMM, stem cell transplant).

Final checklist

• I estimated the dose by biodosimetry and physical dosimetry.
• I recognized the latent phase and did not discharge the patient.
• I started the growth factor early, checking the package insert.
• I ordered a blood count every 3 days and defined the stopping criterion.
• I indicated protective isolation and a plan for febrile neutropenia.
• I ordered irradiated, leukoreduced blood components.
• I collected HLA typing if recovery seemed unlikely.
• I involved hematology and the reference center.
• I explained to the patient what to expect in the coming weeks.

Want to practice reasoning with progressive clues? Play the case of the day.

References

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2. Christy BA, Herzig MC, Wu X, et al. Cell Therapies for Acute Radiation Syndrome. Int J Mol Sci. 2024. PMID 39000080.
3. Farese AM, Cohen MV, Katz BP, et al. Filgrastim improves survival in lethally irradiated nonhuman primates. Radiat Res. 2013. PMID 23210705.
4. Guo M, Dong Z, Qiao J, et al. Severe acute radiation syndrome: treatment of a lethally 60Co-source irradiated accident victim in China with HLA-mismatched peripheral blood stem cell transplantation and mesenchymal stem cells. J Radiat Res. 2014. PMID 23979075.
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13. Radiation Emergency Medical Management. Myeloid Cytokines for Treatment of Acute Exposure to Myelosuppressive Doses of Radiation: Hematopoietic Subsyndrome of Acute Radiation Syndrome (H-ARS). Accessed on 24/10/2026. Official source.
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