The diagnostic problem of atypical Kawasaki disease KD is an acute febrile rash disease with systemic nonspecific vasculitis as the main pathological change. In recent years, the number of cases reported at home and abroad has been increasing year by year, and it has also been found that some cases do not fully meet the diagnostic criteria of KD, but only a few of them, so the concept of atypical or incomplete KD (Atypicalor incomplete KD) has been proposed. Atypical or incomplete KD is basically common in foreign countries. In the past, some people in China called it atypical KD, but most scholars in China recently believe that it is more accurate or appropriate to call it incomplete KD than atypical KD. The incidence of incomplete KD ranges from 10% to 36%, and a recent study in Japan showed that incomplete KD accounted for 13.8%, while two reports with larger samples in China reported incomplete KD of 19.4% and 17.8%, respectively. Understanding and mastering the clinical manifestations and diagnostic points of incomplete or atypical KD is of great clinical significance to minimize or avoid missed, misdiagnosed or delayed diagnosis in the process of K diagnosis and treatment, in order to provide timely medication and treatment, reduce cardiovascular complications and improve the prognosis. Definition of incomplete or atypical KD Incomplete or atypical KD refers to cases with less than five of the six major clinical manifestations of KD, but only three to four of them, that is, cases that have not yet met the diagnostic criteria of typical KD, and have incomplete clinical manifestations but have excluded other diseases. In incomplete or atypical KD cases, the most common manifestation was fever, which was mostly ≥5 d in duration, and in rare cases, no fever. The other 5 clinical manifestations were less frequent than in typical KD, among which the more common manifestations were bilateral conjunctival congestion and dry red chapped lips, while cervical lymph node enlargement, hand and foot sclerosis, and membranous peeling of fingers and toes were relatively less frequent. Polymorphic rash is also uncommon, sometimes transient and short-lived, and may have subsided by the time of presentation, so a detailed history should be taken. These manifestations are present in different combinations in different cases. In addition, these manifestations do not occur simultaneously in a child and may be only partially present at the time of presentation, with other signs and symptoms appearing during treatment and observation. Incomplete or atypical KD is defined as having fewer than 5 major clinical manifestations from the beginning to the end, including the post-subacute phase. As mentioned above, children with incomplete or atypical KD have only a few of the main clinical manifestations of KD and do not meet the diagnostic criteria of typical KD, so it is difficult to establish the diagnosis of KD. When diagnosing incomplete or atypical KD, it is worth noting several issues: ① Certain clinical manifestations can help in the diagnosis of incomplete or atypical KD. In recent years, some clinical observations have shown that during the acute phase of KD, some children have reported that about 50% of them may have perianal or even perineal skin flushing and subsequent desquamation, which is more specific and can help in the early diagnosis of KD; there are also many reports that some children with KD have reappeared erythema at the scar of the original BCG vaccination, and this sign is mostly seen in infants and young children, which also has a greater reference value for the diagnosis of KD; 7 to 15 days after the disease, the nail bed at the end of the fingers and toes moves. This sign is also of great reference value for the diagnosis of KD; 7 to 15 d after the disease, membranous peeling of the nail bed at the end of the fingers and toes is a characteristic feature of KD, with an average incidence of about 50%. Cardiac ultrasonography should be routinely performed in suspected incomplete or atypical KD cases. Cardiovascular damage may occur in some children with KD. Cardiac ultrasonography may reveal coronary artery dilatation or aneurysm, coronary artery echogenicity enhancement, coronary artery lumen irregularity, mitral regurgitation and pericardial effusion. There are many studies showing no significant difference in the incidence of coronary artery disease (CAL) between typical KD and incomplete or atypical KD. In clinical practice, when a child with suspected incomplete or atypical KD is encountered, cardiac ultrasound should be performed promptly, and if necessary, repeatedly. If CAL is found, it will help to diagnose incomplete or atypical KD. Currently, it is advocated that CAL is one of the important diagnostic bases for incomplete or atypical KD. Therefore, routine cardiac ultrasonography is particularly important for the diagnosis of complete or atypical KD. However, it should be emphasized that normal cardiac ultrasound findings cannot completely exclude the diagnosis of KD, because not all cases of KD present with cardiovascular damage. The value of changes in certain laboratory parameters for the diagnosis of incomplete or atypical KD should be emphasized. In recent years, some studies have shown that the changes in laboratory indicators of typical KD and incomplete or atypical KD are basically the same. Although the changes of some laboratory indicators in KD are not specific and can also be seen in other diseases, they still have a greater reference significance, especially when the main clinical manifestations of KD are not fully present, the laboratory results of certain indicators are of more value in determining whether incomplete or atypical KD is diagnosed. Among several laboratory indicators, systemic inflammatory indicators such as C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are more significant. Currently, most scholars in China and abroad believe that if CRP ≥30 mg/L and ESR >40 mm/h, the diagnosis of incomplete or atypical KD should be considered in conjunction with clinical practice. Other laboratory indicators that support the diagnosis of incomplete or atypical KD include: platelets ≥450×109/L, plasma albumin ≤30g/L, anemia, routine blood WBC ≥15×10 9/L, elevated ghrelin, and urine leukocytes ≥10/HP after 7 d of disease, and it is more meaningful to diagnose incomplete or atypical KD if three or more of the above laboratory indicators are present at the same time. ④ Other similar diseases should be excluded before the diagnosis of incomplete or atypical KD. KD is an acute febrile rash disease. In pediatric clinics there are a number of diseases that can present with fever, rash, and swollen cervical lymph nodes, such as scarlet fever, measles, EBV infection, adenovirus infection, Mycoplasma pneumoniae infection, juvenile rheumatoid arthritis (systemic type), and drug allergy syndrome. Identification should be carefully done when encountering specific cases, but sometimes it is difficult and even misdiagnosis can occur. Sometimes dynamic observation and a thorough examination are required to finally confirm the diagnosis. New experts from the American Heart Association have proposed a new guiding rule to help clinicians diagnose incomplete Kawasaki disease: children with fever of unknown origin for ≥5 d who meet only 2 or 3 of the major clinical features of Kawasaki disease should be alerted to the possibility of incomplete Kawasaki disease. First, observe the changes in CRP and ESR. If CRP > 30 mg/L and ESR ≥ 40 mm/h, plus 3 or more other laboratory indices (as mentioned above) are consistent with Kawasaki disease features, incomplete Kawasaki disease can be proposed and echocardiography is required and IV IG therapy is given. If <3 of the other laboratory indices are consistent, echocardiography will be performed first. If the echocardiogram shows coronary artery lesions, the diagnosis of incomplete Kawasaki disease is proposed and IVIG treatment is given. If the echocardiogram does not show coronary lesions and the fever remains, repeat the echocardiogram. If the fever resolves on its own, KD is unlikely. If CRP < 30 mg/L and ESR < 40 mm/h on laboratory tests, close clinical observation should be performed and CRP changes should be detected daily. If there is typical desquamation at the end of the fingers and toes after the fever subsides, the clinical diagnosis of incomplete Kawasaki disease should be made, and echocardiography should be performed to determine whether there is coronary artery dilation. If there is no typical peeling of the fingertips after the fever subsides, Kawasaki disease can be ruled out. The differential diagnosis of Kawasaki disease needs to be made in those children with fever of unknown origin for more than 5 d with any of the main clinical manifestations of Kawasaki disease.