Table 1.

A summary of the prognostic factors associated with platelets in COVID-19.

	Patients No.	Age (Y)	Male (%)	Location	Outcome	Ref
Platelets count
	1476	M: 57	52.6	China	Thrombocytopenia is associated with increased risk of in-hospital mortality, and compared with survivors, non-survivors were more likely to have thrombocytopenia and had lower nadir platelet counts..	[13]
	Non-survivors: 29 Survivors: 503	M: 64 M: 48	48.346.1	China	Early decrease in platelet count was associated with mortality in patients with COVID-19., as the platelet count among non-survivors decreased gradually within 1 week after admission.	[19]
	7613	NA	NA	NA	Thrombocytopenia might be a risk factor for COVID-19 progressing into a more severe state, as compared to non-severe patients, severe COVID-19 had a lower platelet count.	[17]
	178	M: 64	59.6	China	Thrombocytopenia correlated with DIC rate and survival. Six out of 7 deaths had thrombocytopenia during hospitalization, and platelet count decreased subsequently until death.	[8]
	167	M: 66	67.07	China	Thrombocytopenia was associated with the deterioration of respiratory function and baseline platelet count was associated with long-term mortality in critically ill COVID-19 patients.	[12]
	3383	NA	NA	China, Singapore	Decreased number of platelets more commonly associates with severe COVID-19; however, whether thrombocytopenia may result in diseases severity or the severity may decrease platelets is open to debate.	[16]
	1779	NA	NA	China, Singapore	Low platelet count is associated with increased risk of severe disease and mortality in patients with COVID-19, and thus should serve as clinical indicator of worsening illness during hospitalization.	[40]
	215	M: 64	55.81	Turkey	Although thrombocytopenia was more likely occur in non-survivors, there was no correlation between platelet level & mortality.	[20]
	251	M: 8.92	55.5	Turkey	Severe COVID‐19 patients had significantly more common thrombocytopenia than non-severe cases.	[24]
	Non-severe: 71 Severe: 44	M: 48 M: 64	54.92 69	Canada	Platelet counts were in the lower range in both severe and non-severe COVID-19 patients in comparison to the expected count in healthy volunteers; however, the platelet-lymphocyte ratio was lower in severe patients compared with nonsevere patients.	[41]
	74	M: 65.1	59	Iran	Lower platelet count was detected among non-survivors compared to survivors.	[42]
	NA	NA	NA	NA	Thrombocytopenia is a significant finding in patients with severe type of COVID −19. Immune mediated platelet destruction might account for the delayed-phase thrombocytopenia in a group of patients.	[2]
	575	NA	NA	UK	Thrombocytopenia is common in an ICU setting due to endogenous and iatrogenic factors. Despite that, thrombocytopenia in patients with severe COVID-19 infections is surprisingly uncommon.	[4]
	Non-severe: 20 Severe: 34	M: 69 M: 59	65 62	Ireland	While platelet counts did not differ on the day of admission between patients subsequently designated to have had either a severe or non-severe disease course, significantly decreased platelet counts were observed among the severe COVID-19 patients at the time of transfer to the ICU relative to the non-severe group.	[5]
	567	M: 63	52.2	Turkey	Decreased platelet count together with older age, presence of heart failure, clinical severity of the disease at presentation, ferritin level on admission, and increase in AST level during hospitalization may predict the mortality risk of these patients.	[43]
	53	M: 58.4	45.3	Iran	There was an association between higher mortality rates and decreased platelet count.	[44]
	500	M: 44.24	50	India	Platelet count may be a simple, economic, rapid and commonly available laboratory parameter that could straightforwardly discriminate between COVID patients with and without severe disease.	[45]
	64	M: 57.11	47.7	Greece	Platelets were slightly lower in severe patients compared to the moderate ones, but it was not statistically significant.	[46]
	516	M: 67	66.9	Italy	Using Cox regression analysis, platelet count was a predictor of mortality.	[47]
	44	M: 67.5	63.63	Italy	In the univariate analysis for disease severity, thrombocytopenia together with male sex, respiratory frequency greater than 22, and cancer as comorbidity were significantly associated with higher odds for severe disease.	[48]
	110	M: 56.9	43.6	Korea	Lymphocyte count and platelet count were significantly lower in the severe group than the non-severe group.	[49]
	2054	M: 59	52.6	Brazil	In the multivariate Poisson regression model, various factors including low blood platelet count were independently associated with a higher risk of death.	[50]
MPV
	215	M: 64	55.81	Turkey	Oxygen saturation at admission and the MPV difference between the first and third days of hospitalization were significant parameters for predicting mortality.	[20]
	112	M: 61	65.2	China	Patients with high MPV (HR 3.73; P = 0.0034) were significantly associated with the worse survival.	[21]
	85	M: 43	51.8	China	Compared with mild patients, patients with severe pneumonia showed a higher MPR level; proposing high MPR level as an independent risk factor for severe pneumonia in COVID-19.	[22]
	Non-severe: 20 Severe: 34	M: 69 M: 59	65 62	Ireland	Increased MPV and decreased platelet were associated with disease severity in COVID-19 upon hospitalization and intensive care unit admission.	[5]
	37	M: 54	56.8	Turkey	They did not determine difference in MPV level, mortality and prognosis between COVID-19 patients.	[23]
	251	M: 8.92	55.5	Turkey	MPV values are not associated with COVID‐19 disease severity; however, MPV can be used with other parameters such as WBC, CRP, procalcitonin, D‐dimer to predict hospitalization.	[24]
	100	M: 47.1	57	India	High PDW shows significant association with mortality and cut-off values for PDW as 17% significantly associated with mortality.	[51]
	640	NA	NA	Turkey	MPV could be used as a simple and cost-effective tool to predict COVID-19 in subjects with diabetes in primary care.	[52]
	81	M: 60	49	USA	There was a significant correlation between D-dimer levels and MVP; but a negative correlation between MPV and GFR in critically ill cohort.	[53]
IPF
	47	M: 56	59.6	Israel	Patients with COVID-19 have increased IPF compared to stable patients with cardiovascular risk factors, suggesting that the enhanced platelet turnover may have a role in the development of thrombotic events in COVID-19 patients.	[27]
	982	M: 71	68	Europa	IPF tended to increase with disease severity.	[54]
	81	M: 60	49	USA	This study found increased IPF (greater than7) in COVID-19-infected subjects who had evidence of elevated D-dimers and AKI.	[53]
Increased P-selectin level
	Non-severe: 20 Severe: 34	M: 69 M: 59	65 62	Ireland	Circulating levels of soluble P-selectin increased in COVID-19 patients compared to the control cohorts and intriguingly, could differentiate between non-severe and severe COVID-19 cohorts.	[5]
	Non-severe: 71 Severe: 44	M: 48 M: 64	54.92 59	Canada	Platelet activation, assessed by surface expression of P-selectin and CD63, was evident in patients with severe COVID-19 and strongly correlated with levels of D-dimers.	[41]
	68	M: 62	60	USA	Soluble P-selectin concentrations were significantly higher in ICU patients than non-ICU cases.	[31]
	Cohort 1: 60 Cohort 2: 50	M: 58 M: 56	76.7 78	Europa France	Soluble P-selectin was a biomarker for poor outcome and could serve as a soluble marker associated with death. Transcriptional analysis identifed SELPLG RNA level as a biomarker for mechanical ventilation.	[30]
	Survivors: 28 Non-Survivors: 10	X: 62 X: 68	82.1 80	Greece	Soluble P-selectin was significantly elevated in ICU non-survivors compared to survivors, and also associated with a higher mortality probability in the Kaplan–Meier analysis.	[32]
	46	M: 65.2	NA	Italy	A higher P-selectin plasma concentration was found in COVID-19 patients regardless of ICU admission.	[55]
	Healthy: 20 Non-severe: 37 Severe: 64 Non-Survivors: 18	M: 41.40 M: 44.52 M: 55.53 M: 63.77	40 62.16 60.9 50	Mexico	A significant difference was found in P-selectin in non-severe and healthy donors when compared to severe COVID-19 cases and deceased patients. Indee, a higher P-Selectin concentrations was found in the severe and death COVID-19 groups compared with patients in the non-Severe COVID-19 group and healthy donors.	[56]
Leukocyte-platelet aggregation
	Mild: 6 Severe: 35	M: 32 M: 57	33.3 46.8	Brazil	Increased platelet activation and platelet-monocyte aggregate formation are observed in severe COVID-19 patients, but not in patients presenting mild COVID-19 syndrome.	[34]
	27	M: 71	52	France	Levels of NPA and MPA were significantly higher in severe COVID-19 patients relative to those with moderate disease.	[35]
Up- or down-regulation of coagulation factors
	68	X: 62	60	USA	Mortality was significantly correlated with vWF antigen and soluble thrombomodulin among COVID-19 cases. Soluble thrombomodulin greater than 3.26 ng/mL was associated with lower lower likelihood of survival.	[31]
	Survivors: 28 Non-Survivors: 10	X: 62 X:68	82.1 80	Greece	ICU admission levels of Ang-2, sICAM-1, and vWF were higher in COVID-19 critically ill patients who will not survive.	[32]
	183	M: 54	53.55	China	Abnormal coagulation results, especially markedly elevated D-dimer and FDP were common in non-survivors.	[57]
	1621	X: 69	53.2	China	The associations between coagulation factors, vWF and ADAMTS13, and COVID-19 severity are essentially causal; highlighting the importance of dynamically monitoring the plasma levels of these factors in COVID-19.	[37]
	191	M: 56	62	China	D-dimer values were nearly 9-fold higher in patients who died than in those who survived.	[14]
	41	M: 49	73	China	PT and D-dimer level on admission were higher in ICU patients than non-ICU patients.	[58]
	449	M: 65	59.68	China	D-dimer, PT, and age were positively, and platelet count was negatively correlated with 28-day mortality.	[59]
	138	M: 56	54.3	China	The level of D-dimer was significantly higher in non-survivors than in survivors.	[60]
	1099	M: 47	58.1	China	D-dimer was higher in patients with severe COVID-19 than in those without.	[61]
	192	M: 69.4	58.3	Italy	No association was found between lupus anticoagulant and mortality as well as mechanical ventilation.	[62]
	Healthy: 20 Non-severe: 37 Severe: 64 Non-Survivors: 18	M: 41.40 M: 44.52 M: 55.53 M: 63.77	40 62.16 60.9 50	Mexico	Concentrations of D-dimer and plasminogen activator inhibitor-1 were significantly elevated in severe COVID-19 patients. A significant difference was found in PAI-1 in non-severe and healthy donors when compared to severe and deceased COVID- 19 patients. vWF levels were also significantly different between severe patients and non-severe cases.	[56]
	2277	NA	NA	China	Screening abnormal coagulation parameters such as decreased platelet, prolonged PT, and elevated D-dimer were beneficial for predicting the severity and prognosis of COVID-19.	[15]

DIC: Disseminated intravascular coagulation; AST: Aspartate transaminase; MPV: Mean platelet volume; MPR: Platelet mean volume/platelet count ratio; WBC: White blood cells; CRP: C-reactive protein; PDW: Platelet distribution width; GFR: Glomerular filtration rates; IPF: Immature platelet fraction; AKI: Acute kidney injury; ICU: Intensive treatment unit; SELPLG: selectin P ligand; NPA: Neutrophil–platelet aggregate; MPA: Monocyte–platelet aggregate; vWF: von-Willebrand factor; Ang-2: Angiopoietin-2; sICAM-1: Soluble intercellular adhesion molecule-1; FDP: Fibrin and fibrinogen degradation product; ADAMTS13: A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13; PT: Prothrombin time; PAI-1: plasminogen activator inhibitor 1; Y: Year; M: Median; X: Mean; NA: Not available.